04/10/2026
The Rise of Papal Supremacy under Leo the Great and the Rift with Alexandria:
The Path to the Permanent Schism of 451
In the mid-5th century, the Christian Church stood at a crossroads. The major patriarchal sees—Rome, Alexandria, Constantinople, and Antioch—jockeyed for influence amid doctrinal crises, imperial politics, and shifting power dynamics in the late Roman Empire. Pope Leo I (440–461), later called “the Great,” emerged as a pivotal figure who systematically articulated and enforced a doctrine of Roman Petrine supremacy. This development clashed directly with the ancient self-understanding of the See of Alexandria as the preeminent guardian of apostolic orthodoxy. The resulting tension, crystallized in the Christological controversy surrounding Eutyches, produced the Second Council of Ephesus (449, derided by Leo as the “Robber Synod” or Latrocinium) and the Council of Chalcedon (451). While Chalcedon is revered in the Catholic and Eastern Orthodox traditions as the Fourth Ecumenical Council, it provoked the first major permanent schism in Christianity: the separation of the miaphysite (Oriental Orthodox) churches, which trace their roots to Alexandria’s Cyrillian tradition. This article examines Leo’s rise, his assertions of supremacy, Alexandria’s resistance, and how these factors converged to create an enduring divide.
Leo I’s Early Pontificate and the Articulation of Petrine Primacy (440–448)
Leo, a Roman deacon of aristocratic background and significant administrative experience, was elected bishop of Rome on 29 September 440 while absent on a diplomatic mission in Gaul. From the outset, he framed his office in explicitly Petrine terms. In his sermons delivered annually on the anniversary of his ordination (especially Sermons 3, 4, and 5, from the early 440s), Leo taught that Peter—the “rock” of Matthew 16:18—continued to “live and preside” in his successors at Rome. He declared:
“The blessed Peter persevering in the strength of the Rock… has not abandoned the helm of the Church… For he was ordained before the rest… And still to-day he more fully and effectually performs what is entrusted to him… through Whom he has been glorified… Whatever is rightly done and decreed by us… is of his work and merits whose power lives and whose authority prevails in his See.”
This was not mere rhetorical flourish; it was a developed doctrinal claim that authority flowed from Peter as head to the entire body of the Church, with Rome as the indispensable center of unity. Leo insisted that communion with the Apostolic See was essential to orthodoxy and ecclesiastical order.
The first major practical assertion came in the controversy with Hilary (Hilarius) of Arles, metropolitan of Gaul (c. 443–445). Hilary had deposed bishops (including Celidonius of Besançon and Projectus of Fréjus) and exercised broad metropolitan powers without appeal to Rome. Leo convened a Roman synod, reinstated the bishops, and curtailed Hilary’s jurisdiction. In Letter 10 (c. 445) to the bishops of Vienne, Leo grounded his intervention in Christ’s commission to Peter alone:
“The Lord desired that the sacrament of this gift should pertain to all the apostles in such a way that it might be found principally in the most blessed Peter… And he wanted his gifts to flow into the entire body from Peter himself, as if from the head… The Apostolic See—out of reverence for it… has on countless occasions been reported to in consultation by bishops… and through the appeal of various cases to this see, decisions already made have been either revoked or confirmed.”
On 6 or 8 July 445, Emperor Valentinian III issued an imperial rescript (Novella 17) at Leo’s request, formally recognizing Roman primacy across the Western provinces on the basis of Peter’s merits, Rome’s dignity, and the canons of Nicaea. It mandated civil enforcement of papal decisions. This was the first imperial legal endorsement of such sweeping authority and marked a decisive step in centralizing Western ecclesiastical jurisdiction under Rome.
By 446, Leo extended similar claims eastward in Letter 14 to Anastasius of Thessalonica, stating that “the care of the universal Church would converge in the one see of Peter, and nothing should ever be at odds with this head.” These actions and teachings preceded any involvement in the Eastern Christological crisis. Leo was not reacting to heresy; he was proactively defining Rome’s role as the head that guaranteed unity and orthodoxy.
Alexandria: The Self-Proclaimed Guardian of Orthodoxy
Alexandria viewed itself as the intellectual and theological powerhouse of the East. For centuries it had produced towering defenders of the faith: Clement, Origen (despite later controversies), Athanasius (who nearly single-handedly upheld Nicene orthodoxy against Arianism), and Cyril (patriarch 412–444). Cyril’s victory at the First Council of Ephesus (431) against Nestorius of Constantinople had enshrined a strongly unitive Christology—“one incarnate nature of the divine Word”—as the standard of orthodoxy. Alexandria claimed apostolic succession through Mark (Peter’s disciple) and saw itself as the vigilant protector against innovation, especially Nestorian division of Christ’s person.
Under Cyril’s successor Dioscorus (patriarch 444–451), Alexandria continued this role with vigor. Egyptian bishops and monks regarded their tradition as the authentic inheritance of the fathers. Any external attempt to dictate doctrine—particularly from a Western see with a more juridical than speculative style—struck many Alexandrians as presumptuous. Rome’s growing claims appeared not only as jurisdictional overreach but as a younger church presuming to lecture the ancient guardian of the faith. This cultural and theological self-confidence made conflict almost inevitable when Rome intervened in Eastern affairs.
The Eutychian Crisis and the Tome of Leo (448–449)
The flashpoint arrived with Archimandrite Eutyches of Constantinople, who taught an extreme form of Cyrillian Christology that many viewed as blurring or absorbing Christ’s humanity into divinity (later labeled monophysitism). In November 448, a local synod under Flavian of Constantinople condemned and deposed Eutyches. Eutyches appealed to Emperor Theodosius II and to Rome. Leo responded on 13 June 449 with his famous Tome (Letter 28) to Flavian. In clear Latin terminology, Leo affirmed two natures (divine and human) in one person, without confusion, change, division, or separation. He expected this to serve as the authoritative rule of faith.
From Rome’s perspective, this was Peter speaking through his successor. From Alexandria’s viewpoint, the Tome risked reviving Nestorian division by over-emphasizing “two natures” after the union, threatening the Cyrillian emphasis on unity. Dioscorus, aligned with Eutyches and supported by the emperor, saw Rome’s unilateral doctrinal intervention as an infringement on the collegial, conciliar tradition that Alexandria had long championed.
The “Robber Synod” of Ephesus (449) and Open Rift
Emperor Theodosius II convoked the Second Council of Ephesus in August 449, with Dioscorus presiding. Roman legates presented Leo’s Tome, but Dioscorus refused to allow it to be read. The council rehabilitated Eutyches, deposed Flavian (who died shortly afterward from rough treatment), and ignored Leo’s protests. The Roman legate Hilary could only cry “Contradicitur!” (It is contradicted!). Leo, upon receiving reports, denounced the gathering as a latrocinium (“robber synod”) and excommunicated its leaders. The council’s actions deepened the perception in Alexandria that Rome was seizing doctrinal control, while Rome viewed Dioscorus as a tyrant obstructing orthodoxy.
Chalcedon (451): Triumph of the Tome, Deposition of Dioscorus, and the Breaking Point
After Theodosius II’s death, his sister Pulcheria and Emperor Marcian convened the Council of Chalcedon in October 451 at Leo’s urging. Over 500 bishops attended. In Session II, after the Tome was read, the bishops acclaimed: “Peter has spoken through Leo!… Leo and Cyril taught the same thing!” Yet many Eastern bishops remained uneasy. Dioscorus was tried and deposed—initially for disciplinary offenses (failing to appear at three summons and for his conduct at Ephesus II), though theological differences loomed large.
The council accepted Leo’s Tome as orthodox and issued a definition affirming “one and the same Christ… acknowledged in two natures, without confusion, without change, without division, without separation.” This was a clear dyophysite formulation. In a later session, the bishops passed Canon 28, elevating Constantinople to second place after Rome and granting it jurisdiction over certain provinces—directly challenging Alexandria’s ancient prerogatives (second after Rome per Nicaea). Leo’s legates protested vehemently; Leo later annulled the canon by his own authority, declaring it void because it violated Nicaea and diminished Alexandria and Antioch.
For Alexandrian traditionalists, Chalcedon felt like a coordinated assault: Rome’s Tome redefined orthodoxy in Western terms, Dioscorus (their patriarch) was deposed, and Constantinople was exalted at Alexandria’s expense. Egyptian bishops largely rejected the council. When Proterius (Dioscorus’s replacement) was imposed, riots erupted. The miaphysite party—loyal to Cyril and Dioscorus—elected their own hierarchy, leading to the permanent separation of what became the Oriental Orthodox (Coptic, Syrian, Armenian, etc.) churches. They viewed Chalcedon as a betrayal of Cyrillian orthodoxy and a Roman power grab.
The Enduring Rift: Theological, Jurisdictional, and Cultural
The schism was not solely about Christology. It fused two irreconcilable visions:
Rome: Primacy of the Apostolic See as the head from which unity and orthodoxy flow (Petrine doctrine systematized by Leo).
Alexandria: Collegial conciliarity and fidelity to the Cyrillian tradition as the authentic voice of the fathers.
Leo's pre-449 assertions of supremacy had already signaled Rome’s intent to exercise universal oversight. The crisis revealed that Alexandria would not yield its historic role as orthodoxy’s intellectual defender. Imperial politics exacerbated the divide, but the core issue was ecclesiological: who defines and enforces doctrine? The miaphysite churches never accepted Chalcedon or Leo’s Tome, preserving their identity as the true heirs of Cyril and the pre-Chalcedonian faith. The wound, though later softened by ecumenical dialogue, remains unhealed to this day.
Leo’s pontificate thus marked both the consolidation of Roman primacy in the West and the first irreversible fracture of the universal Church. What began as sermons on Peter’s enduring headship ended in a schism that reshaped Christendom. The events of 449–451 demonstrated that doctrinal unity could not be imposed unilaterally when ancient sees like Alexandria felt their apostolic legacy under threat. The rise of papal supremacy under Leo the Great came at the cost of the Church’s visible oneness.
