De Pelagiana Haeresi, S Leonis Magni Aevo Gliscente (c.445)
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This historical-theological compilation documents Pope Leo the Great's decisive campaign against Pelagianism in 5th-century Italy, demonstrating how his papal authority consolidated Augustine's doctrines on grace, original sin, and salvation throughout the Western Church.

Pelagius – a 5th-century monk from Britain – whose teachings gave rise to Pelagianism, a heresy denying original sin and the need for divine grace.
Pelagianism takes its name from Pelagius and was a 5th-century movement that sharply opposed the Augustinian doctrine of grace. It denied the doctrine of original sin and the necessity of Christian grace for salvation, asserting that human nature was essentially unfallen and that individuals could achieve righteousness by the natural power of free will ewtn.com. In Pelagius’s view, Adam’s sin harmed only Adam, not the human race; newborns are in the state Adam was before the fall, and baptism is not needed to erase any inherited sin. Crucially, Pelagians taught that God’s grace is not indispensable – grace was seen more as an external help (like law and teaching) or given according to personal merit, rather than a free, unmerited gift necessary to empower any good act sacred-texts.com sacred-texts.com.
Pelagius began promoting these ideas in Rome around c. 400–410, alongside disciples like Celestius and later Bishop Julian of Eclanum. They were alarmed by what they perceived as moral laxity attributed to Augustine’s emphasis on grace. Pelagian theology held that moral perfection is attainable in this life without special grace, since human free will itself was capable of choosing good or evil without the impairment of a hereditary sin nature. This implied that infants dying unbaptized could attain eternal life, and that the role of Christ’s grace was to enlighten and instruct, rather than to internally heal a corrupted nature. The theological implications were far-reaching: Pelagianism effectively nullified the need for a savior’s atoning grace, since humans, untouched by Adam’s fall, could theoretically live sinlessly. Such views clashed directly with orthodox teaching on the Fall, baptism, and grace. St. Augustine of Hippo emerged as Pelagius’s chief opponent, arguing that Pelagian ideas made Christ “die in vain” since, if human nature could secure holiness on its own, the redemptive grace of Christ would be superfluous. Augustine and other African bishops condemned Pelagian doctrines in local councils, stressing that Adam’s guilt and a wounded will have affected all humans, so that we absolutely depend on prevenient grace for any meritorious good ewtn.com sacred-texts.com.
Origins and condemnation: Pelagius’s teachings were first scrutinized in 415 at the Synod of Diospolis (Lydda), where he managed a qualified acquittal. But outrage in North Africa led to Councils at Carthage and Mileve (416), which condemned Pelagian tenets and appealed to Pope Innocent I. By 418, a broader consensus was reached. A Carthage council in 418 issued canons rejecting Pelagian claims (e.g. that even unbaptized infants have eternal life, that grace merely makes doing good easier, not essential). Pope Zosimus, after initially wavering, confirmed Pelagius’s and Celestius’s excommunication in 418. The Emperor Honorius buttressed this with an edict banishing Pelagians from Italy ewtn.com ewtn.com. Thus, Pelagianism was formally condemned as heresy throughout the Western Church. However, as contemporary observers noted, it was “condemned, but not crushed” ewtn.com. Pelagius himself fades from history after 418, but Julian of Eclanum and others continued to defend Pelagian ideas, keeping the controversy alive into the mid-5th century. It is in this context – with Pelagianism officially anathema but still “gliscente” (surfacing here and there) – that Pope Leo I the Great confronted the heresy during his clerical career.
Leo the Great on Grace, Free Will, and Original Sin
Pope St. Leo I (“Leo the Great”), who reigned from 440–461, emerged as a staunch defender of Augustinian orthodoxy on grace and original sin. Even before becoming pope, Leo had distinguished himself in the fight against Pelagianism. As a deacon in Rome under Popes Celestine and Sixtus III, Leo was instrumental in thwarting Pelagian attempts at rehabilitation. Prosper of Aquitaine records that around 439, the exiled Julian of Eclanum tried to regain his episcopal seat by professing orthodox faith. “No one deceived Julian except Saint Leo, then archdeacon of the Roman Church,” who urged Pope Sixtus III to reject the heretic’s feigned repentance la.wikisource.org. Thanks to Leo’s vigilance, “Pope Sixtus, at the exhortation of Deacon Leo, allowed no entry to these pestilential schemers,” and Julian’s ruse was foiled – an outcome that made all Catholics rejoice as if “the apostolic sword had [again] cut down that most proud heresy” la.wikisource.org. This incident highlights Leo’s early commitment to safeguarding the true doctrine of grace.
Upon becoming Bishop of Rome in 440, Leo the Great continued to combat Pelagian tendencies. He articulated a clear doctrine on grace, free will, and original sin that aligned with Augustine’s teachings. Leo emphasized the necessity of divine grace at every stage of salvation, in harmony with human free will rightly ordered. In one letter, he condemns the Pelagians’ subtle tactic of affirming grace in name but emptying it of substance: they pretend to disapprove their errors but secretly insist “that the grace of God is given according to the merits of the recipient.” Leo refutes this, noting that if grace is earned, “unless it is given freely, it is not a gratuity but a payment for merits” sacred-texts.com. He quotes St. Paul: “By grace you have been saved through faith, and that not of yourselves: it is the gift of God, not of works…” (Eph 2:8–9) sacred-texts.com. For Leo, as for Paul and Augustine, salvation is God’s free gift, not a reward for prior human goodness.
Leo also upheld the Church’s teaching on original sin. He pointed out that Pelagian doctrine was constructed precisely to avoid acknowledging humanity’s fallen state: Pelagians posited a “natural goodness” in man that existed before grace and was not marred by any stain of original sin, effectively “crying down original sin” sacred-texts.com. Against this, Leo asserted the traditional dogma: Adam’s sin has wounded human nature, and Christ’s grace is absolutely needed to heal that wound. Echoing Jesus’ words, Leo writes that by denying mankind’s lost condition, Pelagians falsify “what the Truth says: ‘For the Son of Man came to seek and to save what was lost’.” sacred-texts.com In Leo’s theology, human free will is affirmed – people must freely cooperate with grace – but such freedom itself must be liberated and elevated by God’s grace. Prideful reliance on free will alone leads only to error and “boasting.” Leo stresses that Christian humility requires acknowledging that even our good will is prepared by the Lord: “[God’s] grace is to everyone the source of righteousness… the fountain of merit”, not something we earn by our own powers sacred-texts.com.
In sum, Leo the Great taught a balanced synergy of grace and free will: man’s will is free, but enslaved by sin unless freed by grace. All good works are initiated by God’s grace and require human consent. This stance directly opposed Pelagian “perfectionism” and self-reliance. Leo’s pastoral writings show him defending the necessity of infant baptism (since even infants need the regeneration of grace) and the ongoing need for God’s help in the life of the baptized. In one sermon he declares that “whoever glories in anything of his own, rather than in the Lord, is deceiving himself”, reflecting his view that all credit for virtue belongs to God’s grace, not to autonomous human effort (a clear rejection of Pelagian pride) sacred-texts.com sacred-texts.com. Thus Leo stood firmly in the tradition of Augustine – so much so that later theologians have considered Leo’s anti-Pelagian pronouncements a concise restatement of Augustinian doctrine for mid-5th-century Rome.
The Church’s Response to Pelagianism in Leo’s Era
By the time Leo became pope, Pelagianism had been formally condemned but remnants of the heresy still lurked, especially in parts of Italy. Leo’s pontificate saw an energetic institutional response to eradicate these lingering Pelagian influences. Early in his reign (c. 442), Leo learned from Bishop Septimus of Altinum that in the province of Aquileia (northeast Italy), clergy who had previously been Pelagians or “Celestians” were being admitted back into communion without any formal renunciation of their errors catholicculture.org catholicculture.org. Shocked that “wolves in sheep’s clothing” had been allowed to slip in while the local bishops “were fast asleep,” Leo took decisive action catholicculture.org. He sharply rebuked the negligent church authorities and, by authoritative decree, ordered a provincial synod to set matters right catholicculture.org newadvent.org. All clergy rejoining the Church who had been aligned with Pelagianism were required to make a public recantation of their false beliefs. Leo specified that they must “condemn the authors of this presumptuous error [Pelagius and Celestius]” and sign a full, unequivocal confession of the true faith, endorsing all the synodal decrees by which the Apostolic See had previously repudiated Pelagian doctrine sacred-texts.com sacred-texts.com. Any ambiguity or doctrinal evasiveness was to be rejected – Leo knew the Pelagians were clever in concealing their ideas, so he insisted on absolute clarity (no “obscure or ambiguous” language in their professions) sacred-texts.com sacred-texts.com.
Leo’s instructions to Aquileia’s bishops combined doctrinal firmness with disciplinary measures. He ordered that those who refused to comply with the required abjuration be excommunicated and expelled from ecclesiastical office sacred-texts.com sacred-texts.com. Furthermore, Leo reinforced earlier canons barring clergy from roaming between dioceses – a practice Pelagian sympathizers had used to spread their ideas under the radar sacred-texts.com sacred-texts.com. By insisting that clerics remain under their own bishop’s supervision, Leo hoped to contain the contagion and prevent expelled Pelagians from finding refuge elsewhere.
These actions demonstrate the institutional resolve of the Church in Leo’s era: a heresy that had been anathematized by councils was not to be treated laxly at the local level. Thanks to Leo’s intervention, a council at Aquileia (circa 443) did take place, and those suspected of Pelagianism either formally recanted or were removed. The Catholic Encyclopedia summarizes that Leo “directed that a provincial synod be assembled in Aquileia, at which such persons were to publicly abjure Pelagianism and subscribe to an unequivocal confession of Faith” (letters Ep. 1 and Ep. 2) newadvent.org. Leo’s vigilance “put a stop to the growing evil” of Pelagian teaching in that region ecatholic2000.com.
Beyond Italy, Leo’s papacy maintained pressure against Pelagianism wherever it flickered. In southern Gaul and Spain, so-called “Semi-Pelagian” tendencies (a moderated form, teaching the initiative of the human will in conversion but affirming the necessity of later grace) were present among some monastic circles. Leo was aware of these debates (which had involved figures like St. John Cassian and St. Vincent of Lérins in the earlier decades). While the Semi-Pelagian controversy would be definitively settled only in 529 at the Council of Orange, Leo took steps even in his time to ensure a consistent teaching. Notably, as a deacon he had encouraged John Cassian – who had once been criticized by Prosper for semi-Pelagian leanings – to speak out against heresy. Around 430 Cassian dedicated to Leo a treatise attacking Nestorianism; at Leo’s urging, Cassian’s Seven Books on the Incarnation also “in no way indulged the Pelagians, but assailed and cut them to pieces.” la.wikisource.org Leo essentially co-opted Cassian into the anti-Pelagian fight, ensuring that even the more moderate Eastern monks rallied against Pelagius’s core errors. This demonstrates how Leo’s anti-Pelagian strategy extended beyond formal councils to shaping theological discourse: he leveraged the influence of respected monastic theologians to refute Pelagian ideas on a spiritual and intellectual level.
In Rome itself, Pelagianism had largely gone underground by Leo’s tenure – but another heresy, Manichaeism, concerned the pope greatly (and he preached and legislated against it extensively). Still, Leo did not lose sight of Pelagianism. In 445, when writing to Bishop Turibius of Astorga about various Spanish heresies, Leo included Pelagianism among the errors to watch for (alongside the Priscillianist and Manichaean sects) americaneedsfatima.org ecatholic2000.com. We find Leo consistently marshalling the authority of the Apostolic See to support other bishops in extirpating Pelagian teaching. His letters often remind correspondents that Pelagianism had been condemned by his predecessors and the universal Church, giving local pastors both the doctrinal clarity and the legal mandate to act decisively.
This coordinated, hierarchical response under Leo’s leadership was instrumental in finally breaking the back of Pelagian influence in the Western Church. Indeed, a later chronicler could triumphantly report that Leo’s “divine zeal in overthrowing the Pelagians and abolishing their errors obtained the desired end.” After Leo’s efforts, nowhere do contemporary sources speak of the Pelagian heresy as still spreading in the mid-5th century la.wikisource.org. The residual “Pelagian brood” in Italy was effectively “almost destroyed” by about 456 la.wikisource.org la.wikisource.org. Orthodoxy, as articulated by Augustine and enforced by Leo, had prevailed—at least in Leo’s part of the world.
Leo’s Place in the Anti-Pelagian Trajectory (Augustine’s Legacy)
Leo the Great stands as a critical link in the broader trajectory of anti-Pelagian theology, effectively cementing the Augustinian heritage in the teaching of the Western Church. By Leo’s time, St. Augustine (d.430) had left a formidable legacy of writings against Pelagius – works like On Nature and Grace, On Original Sin, and On the Grace of Christ. Leo absorbed this legacy wholeheartedly. In his letters and decrees, Leo frequently echoes Augustine’s ideas (sometimes even Augustine’s phrases) on humanity’s total dependence on grace. While he did not engage in speculative nuances of predestination in the way Augustine eventually did, Leo championed Augustine’s core insights: that Adam’s fall wounded all humanity, that baptismal grace is essential for salvation, and that even the first steps of faith are God’s gracious gift.
The continuity is evident – for example, Leo taught that even the desire to be cleansed comes from the initiative of Christ (an idea Augustine expounded as “Prevenient Grace”). In confronting Pelagians, Leo thus acted as a guardian of Augustine’s doctrine, translating it into papal directives and pastoral actions.
At the same time, Leo was keenly aware of contemporary developments and potential over-corrections. In southern Gaul, some theologians (reacting to Pelagianism) had espoused extreme predestinarian views that virtually denied free will. Although this would be formally addressed after Leo’s death, Leo anticipated the need to steer a middle course. By encouraging figures like Prosper of Aquitaine – a disciple of Augustine who fought Pelagianism but also opposed fatalistic interpretations – Leo helped shape a moderated Augustinianism that would later be ratified by the Church. Prosper, who served in Rome under Leo, likely had Leo’s support in composing materials against Semi-Pelagian arguments. One medieval chronicle even credits Prosper with writing pamphlets that “easily scattered and extinguished” a Pelagian resurgence in Rome itself during Leo’s papacy la.wikisource.org (though Leo’s own involvement is highlighted). In effect, Leo provided authoritative backing to the anti-Pelagian campaign that Augustine’s followers continued.
Leo’s papacy came roughly a decade after the Council of Ephesus (431), which not only condemned Nestorius but also – at the insistence of Augustine’s supporters – reiterated the Church’s condemnation of Pelagianism. The Acts of Ephesus had included Pelagianism in the list of heresies rejected by the ecumenical council. Leo, fully supportive of Ephesus’s decrees, saw himself as enforcing an already settled matter. His contribution was to ensure the reception of Augustine’s theology into active Church discipline. Under Leo, Augustine’s anti-Pelagian teachings passed from North African councils into Roman practice and canon law. For example, the requirement that returning Pelagians sign on to “all the synodal decrees” against the heresy sacred-texts.com shows Leo implementing the canons first drafted by Augustine and the Africans in 418. In this sense, Leo can be seen as Augustine’s executor on the papal throne: he preserved Augustine’s legacy at a time when the Western Empire was crumbling and the intellectual leadership of North Africa had been silenced (Roman North Africa fell to the Arian Vandals in 439, shortly after Augustine’s death). Leo ensured that Rome became the new center of anti-Pelagian orthodoxy.
It is telling that later Councils and debates consistently invoked Leo alongside Augustine. The Second Council of Orange in 529, which definitively condemned Semi-Pelagianism, reflected principles that Leo had upheld – for instance, Orange taught that the beginning of faith (even the desire to be baptized) is a gift of grace, a claim Leo had made against the Pelagians. Medieval commentators, looking back, regarded Leo as a Doctor who helped solidify the Augustinian doctrine of grace in the official teaching of the Church. By confirming that Pelagianism was not merely an African concern but a heresy Rome itself must guard against, Leo universalized Augustine’s legacy. In short, St. Leo I functioned as an authoritative interpreter of Augustine for the post-Augustinian generation. He made the victory over Pelagianism concrete and permanent by welding it to the authority of the Papacy. Later theologians like St. Prosper and St. Fulgentius of Ruspe owed much to Leo’s precedent as they further expounded grace and free will.
Leo also interacted with monastic spirituality in the Pelagian context. A notable example is his relationship with John Cassian mentioned above. Cassian had been suspected by some of Semi-Pelagian sympathies due to his emphasis on free will in his Conferences. By exhorting Cassian to write against Nestorius and pointedly ensuring Pelagian errors were attacked in that work, Leo effectively rehabilitated Cassian’s reputation on grace la.wikisource.org la.wikisource.org. In the dedicatory letter, Cassian even defers to Leo’s judgment on these matters, indicating Leo’s influence on aligning monastic theologians with Augustinian orthodoxy.
This careful diplomacy prevented a rupture between the pro-Augustine party and more moderate Eastern-influenced monks – which in turn helped the Church eventually forge a consensus at Orange, integrating truth from both sides. Thus, Leo’s role was not only as a doctrinal enforcer but also as a bridge-builder who helped integrate the wider Christian tradition with Augustine’s insights, ensuring Pelagianism in all its forms was refuted without alienating orthodox believers who struggled with some of Augustine’s harsher expressions. In doing so, Leo set the stage for a more nuanced development of grace theology after his time, all while holding the line against any revival of Pelagian thought.
Leo’s Writings and Sermons Engaging Pelagian Themes
Leo the Great’s extant writings include 173 letters and around 100 sermons. Among these, a few key letters directly address Pelagianism and related issues, giving us insight into Leo’s approach. Letter I (written in 442/443) to the Bishop of Aquileia is the most explicit. In this letter, Leo identifies the spread of “the Pelagian heresy” in Aquileia and lays out a program for correction catholicculture.org catholicculture.org. He expresses alarm that former Pelagians were welcomed without renouncing their “peculiar error,” and he deploys vivid metaphors – the clergy had been asleep while wolves in sheep’s clothing entered the fold catholicculture.org. Leo then systematically instructs the bishop to convene a synod, extract a public abjuration from the errant clerics, and have them anathematize Pelagian doctrines and authors in writing sacred-texts.com sacred-texts.com.
This letter even contains a theological exposition (Section III of the letter) where Leo dismantles the Pelagian view of grace. He notes how the heretics “make exception of the dogma that the grace of God is given according to merits” and counters it with Scripture and logic – if grace is not free, it isn’t grace at all sacred-texts.com sacred-texts.com. He also unmasks their motive: to deny original sin so that human nature’s “good desires” would seem unimpaired and Christ’s mission to save the lost appears optional sacred-texts.com. In Letter II (to Bishop Septimus of Altinum), Leo similarly warns that caution is needed in reconciling ex-Pelagians and that clergy must not be allowed to wander (since some Pelagians had tried to move dioceses to avoid detection) sacred-texts.com sacred-texts.com. These letters, couched as authoritative papal directives, directly engage Pelagian themes – grace vs. merit, the nature of original sin, and Church discipline regarding heresy – and they cite earlier anti-Pelagian councils, effectively synthesizing prior teaching with Leo’s own pastoral mandate.
In addition to these official letters, Leo’s sermons sometimes implicitly touch on Pelagianism by emphasizing human dependence on God’s grace. For example, in his famous Nativity (Christmas) sermons, Leo frequently highlights that “without Christ, mankind could not be saved” and that Christ’s birth is the remedy for the entire human race’s sinful condition – statements that counter any notion of natural human perfectibility. In one sermon, Leo exclaims that “the heights of divine grace have lifted us up to where we are” and that no one should boast in themselves. While he does not name Pelagius in such homilies (these were preached to general audiences), the themes of humility, grace, and the need for divine help run strongly through his preaching, reflecting the same theology he wields overtly against Pelagians in his letters. In a Lenten sermon, Leo teaches that even the desire to do penance is God’s gift and that Christians must attribute their good works to God – a clear echo of anti-Pelagian spirituality, reinforcing that virtue is a result of grace-enabled effort, not unaided will.
It is also worth noting a work associated with Leo titled “De Pelagiana Haeresi” (On the Pelagian Heresy). In some collections (particularly in the Patrologia Latina, vol. 55), this appears as a distinct treatise attributed to Leo. This text is not a single sermon or letter, but rather a compiled historical-theological tract on the Pelagian heresy in Leo’s time. It recounts the history of Pelagianism and Leo’s actions against it, often quoting sources like Prosper and Augustine. For instance, it narrates Julian of Eclanum’s downfall and Leo’s role in it, and it describes how Leo’s “incomparable zeal” uprooted Pelagianism from Italy la.wikisource.org la.wikisource.org. It even includes, as we saw, the story of Leo spurring John Cassian to refute Pelagian ideas la.wikisource.org. This “De Pelagiana Haeresi” reads like a summary of Leo’s anti-Pelagian efforts and contains theological reflections consistent with Leo’s letters (e.g. it celebrates the triumph of grace over the “Pelagian plague”).
Modern scholars have determined, however, that this treatise was likely compiled after Leo’s lifetime – it is commonly classified as Pseudo-Leo. Internal evidence (such as references to later figures and even citations of the historian Livy in these opuscula) suggest an author drawing on Leo’s legacy rather than Leo himself cambridge.org. In all probability, the “De Pelagiana Haeresi” text included in Leo’s works is the product of later editors (possibly the 17th–18th century scholars who edited Leo’s corpus) who wanted to provide a thorough account of Pelagianism “as it was rising in the age of Leo the Great.” It serves as a useful compendium, gathering Leo’s letters, snippets of Prosper, and other materials into one narrative. But while it may not be an authentic work from Leo’s own pen, it draws directly on Leo’s authentic writings, and thus accurately reflects the content of Leo’s teaching and the historical facts of his campaign against Pelagianism. In any case, Leo’s genuine writings – especially his letters 1 and 2 – remain the primary sources where he explicitly engages Pelagianism by name. Those letters had an enduring authority, often being cited by later churchmen dealing with semi-Pelagian debates. For example, Faustus of Riez and Caesarius of Arles (6th century) knew of Leo’s strong stance, and Pope Gelasius I (492–496) followed Leo’s line in insisting that heretics not be restored without clear abjuration of error.
Reception and Legacy of De Pelagiana Haeresi and Leo’s Anti-Pelagian Efforts
The historical reception of Leo’s work against Pelagianism – including the treatise De Pelagiana Haeresi – has been largely positive, solidifying Leo’s reputation as a champion of orthodoxy. In the decades immediately after Leo, the apparent disappearance of overt Pelagian teaching was a testament to the effectiveness of his measures. A contemporary in the late 5th century could write that Leo’s struggle had achieved its goal: in the records of the 5th and 6th centuries, “nowhere do we find mention of the Pelagian heresy as if then growing.” la.wikisource.org The heresy had been so thoroughly checked that it ceased to trouble the mainline churches of the West for a time. Pope Gelasius I in 496 ranked Pelagianism among the deadly heresies definitively condemned by the Church, doubtless relying on the actions of his predecessor Leo to say so with confidence.
That said, Pelagianism did see a small revival decades later, but significantly, only in places beyond Leo’s immediate reach. In the British Isles, isolated from Rome’s direct influence, Pelagian preaching re-emerged in the late 5th and 6th century. Prosper of Aquitaine notes that British bishops requested aid, leading to missions by St. Germanus of Auxerre (in 429 and again c.447) to combat Pelagian preachers in Britain – these missions were contemporary with Leo (and indeed possibly encouraged by Popes like Celestine and Leo). Long after Leo’s death, around 640 A.D., Pope John IV wrote to the Irish bishops about a renewal of Pelagianism in Ireland. He explicitly cited earlier authorities and warnings to correct those errors la.wikisource.org. This shows that Leo’s anti-Pelagian legacy endured: later popes confronting similar problems would ground their responses in the doctrinal clarity that Leo (and Augustine) had established. In his letter, John IV even echoes Leo’s no-nonsense approach, condemning Pelagian tenets that were cropping up among the “Scots” (Irish). Thus, Leo’s impact was felt in how the Church later recognized and dealt with Pelagian tendencies wherever they appeared.
The treatise De Pelagiana Haeresi itself, as included in collections like Migne’s Patrologia Latina, became a reference for medieval scholars. It was essentially a ready-made historical dossier on Pelagianism in Leo’s time. Medieval compilers and Renaissance humanists who published the works of the Church Fathers preserved this text under Leo’s name. By the modern era, scholars identified it as likely pseudonymous (Pseudo-Leo), given its editorial nature cambridge.org. Nevertheless, its content was valued. It provided later generations with a comprehensive narrative of how the “Heresy of Pelagius” was dealt with by Pope Leo the Great. In doing so, it cemented Leo’s image as a vigilant guardian of orthodoxy. In the 19th century, when critical editions were made, editors like the Ballerini brothers included long dissertations on Pelagianism appended to Leo’s letters, further examining Leo’s role. The De Pelagiana Haeresi we have today likely incorporates those learned commentaries. Modern historians (such as in the Cambridge studies) note that works like De Manichaeorum Haeresi and De Pelagiana Haeresi in PL 55 are not truly Leo’s, “but these works are” compilations by later authors drawing on his era cambridge.org. This scholarly clarification doesn’t diminish Leo’s legacy; rather it shows the care with which later centuries documented his battle against heresy.
In Catholic tradition, Leo the Great is venerated not only for his pivotal role at the Council of Chalcedon (451) in defining Christology, but also for his defense of the doctrine of grace. The Council of Trent (16th century), while primarily addressing Reformation issues, indirectly drew on the Augustinian heritage that Leo had helped enshrine – for instance, Trent’s decrees on original sin and justification align with the anti-Pelagian consensus to which Leo contributed. In Catholic commemorations, Leo is hailed as a Doctor of the Church; his writings on grace and free will form part of that doctrinal treasury. The legacy of Leo’s anti-Pelagian work is ultimately the Church’s firm teaching that we are saved “by grace, and not by ourselves”, a principle that has remained a cornerstone of Catholic soteriology.
Finally, the very notion of “Pelagianism” as a recurring temptation in Christian thought – the idea that humans might try to earn salvation by their own efforts – owes much to the clarity given by Augustine and Leo. In every age when “Pelagian” attitudes reappear (even unknowingly), Church theologians often invoke the authority of Augustine and Leo to refute them. For example, recent Catholic catechetical texts explicitly mention Pelagianism as condemned, implicitly standing on the foundation laid in the 5th century. In that foundational struggle, Leo the Great’s role was decisive: he provided the effective leadership and doctrinal synthesis that ensured Pelagianism would be remembered as a defeated heresy, not an ongoing option for orthodox Christians. The work De Pelagiana Haeresi – whether directly Leo’s or a later tribute – stands as a testament to how thoroughly the Church of Leo’s time understood and opposed Pelagian doctrine, and how that victory was recorded for posterity. Leo’s name remains forever linked with the triumph of the teaching of grace over the Pelagian heresy in the Western Church.
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