LEO I (d. 461), pope of the Roman Catholic Church (440-461), called "the Great." Nothing is known for certain about Leo's early life, although according to the Liber pontificalis, he was born in Tuscany probably at the turn of the fourth to the fifth century. Leo is one of the most important Roman pontiffs and one of the architects of papal authority. He served as a deacon of the Roman church under both Pope Celestine I (422–432) and Pope Sixtus III (430–440), and in that position exercised great influence. He took an active role in the theological controversies with the Nestorians and the Pelagians and was also involved with institutional matters. While Leo was on a mission to Gaul in early 440, Sixtus died, and the legate returned to Rome to find himself elected pope. He was consecrated as bishop of Rome on September 29, 440.
The energy that Leo had devoted to religious questions before he became pope carried into his pontificate. In the first decade of his papacy, Pelagians, Manichaeans, and Priscillianists were at different times condemned in his writings and even in public debate. Of Leo's undoubtedly extensive homiletical and epistolary production, only 96 sermons and 123 indisputably authentic letters survive. Yet even this legacy is unusually large for a pope in antiquity and permits insight not only into Leo's papal activities, but also into his ideas and beliefs.
Leo considered himself to be, as bishop of Rome, the successor of Peter in a transhistorical sense. When Leo spoke it was the apostle who spoke. Just as all bishops are responsible for the care of their own flocks, so, in Leo's conception, the successor of Peter in the Roman church is charged with the care of all churches, for it was to Peter that Christ gave the keys of binding and loosing in heaven and on earth (Mt. 16:16-19). Just as it was for Peter's faith alone that Christ prayed when all the apostles were threatened (Lk. 22:32), so firmness to the apostolic tradition of the Roman church will strengthen all bishops. With a strikingly deep sense of the traditions both of his office and of Roman law, and with a conviction about the presence of apostolic authority in his words and actions, Leo stands out among other fourth- and fifth-century architects of papal claims, such as Celestine I and Damasus I (366–384). With deft use of such a dossier, it is small wonder that notions such as the contrast between plenitudo potestatis ("fullness of power") and pars solicitudinis ("part of the responsibility")—terms that emerge from a letter of Leo's to Anastasius, his vicar in Illyricum—over time became central in the tradition of describing the powers of Rome vis-à-vis other churches.
Leo was not, however, merely a theoretician of papal claims. He was deeply committed to effective action, whether in a pastoral role at Rome or in the larger sphere of empirewide ecclesiastical politics. He promoted the claims of Roman authority in various ways, whether negotiating in the West with barbarian invaders, or dealing with issues in regions as far removed as Egypt and Gaul. In the former instance, although he acknowledged Dioscorus as successor of Cyril on the patriarchal throne of Alexandria, Leo urged uniformity between the two churches in certain liturgical practices. Tradition stated that the evangelist Mark had founded the Alexandrian church, and Mark was a disciple of Peter, who had "received the apostolic principate [apostolicum principatum ] from the Lord, and the Roman church preserves his teachings" (Regesta pontificum Romanorum, JK406). Leo reasoned that teacher and disciple ought not to represent disparate traditions.
The pope asserted papal authority in Gaul in the face of staunch opposition. The archbishop of Arles had been granted a primacy over the Gallican church by Pope Zosimus I (417–418). The vigorous exercise of that privilege and the objection of local churchmen gave Leo an opportunity to have recourse to the prerogatives of the Roman church. The pope restored to Besançon a bishop who had been deposed by Bishop Hilary of Arles and was able to gain support from Emperor Valentinian III against Hilary. When the latter challenged Leo's authority, the pontiff had him confined in 445 to his diocese by an imperial decree in which the primacy of the bishop of Rome was acknowledged.
The most famous instance in which Leo's claims were manifest involved the renewed Christological dispute in the East in the 440s. When the troubles over Eutyches began at Constantinople, Leo felt that they should have been referred to Rome at once. In 449 the pope sent to Bishop Flavian of Constantinople his famous Tome in the custody of legates destined for the synod held at Ephesus, a synod that Leo later condemned as a latrocinium (a band of robbers, or an act of banditry), rather than a concilium. The problem with Ephesus as Leo saw it was that the gathering was controlled by Dioscorus of Alexandria and concluded by condemning Flavian and rehabilitating Eutyches and his Alexandrian Monophysite Christology.
The events of 449 were reversed through the concerted efforts of Leo, in association with powerful allies in Constantinople, both in the imperial household and in the church. The Roman pontiff's legates and Tome had been ignored in Ephesus. When a new synod was convened at Chalcedon in 451 by the recently elevated emperor Marcian, the opponents of the Alexandrians were firmly in charge. Leo's Tome was received, to quote Henry Chadwick, "with courteous approval" (The Early Church, 1967, p. 203), and it became the basis of the Chalcedonian definition of faith (not a new creed, in deference to the tradition that no faith different from that of the Council of Nicaea, 325, should be proclaimed). The definition set forth a Christology of two natures, divine and human, in Christ, within one person, and represented a triumph for Western views and Roman authority within the complex Eastern world. Chalcedon also, in its twenty-eighth canon, which was enacted without the approval of the Roman legates, elevated the see of Constantinople to a rank in ecclesiastical dignity equal to that of Rome. The pope was furious, refused to accept this decree into the Latin canonical tradition, and even delayed affirming the council's theological decisions.
With Leo, Roman ecclesiastical authority became both a concept and a force to be taken seriously in the Christian world. Scholars debate the extent of his contributions to the sacramentary that bears his name, although Leo may have composed some of the material. There can be no question, however, of Leo's contribution to the development of the papacy as a religious and political force. Together with Damasus I, Gelasius I, and Gregory I, he stands out as one of the most important Roman pontiffs of antiquity, and throughout papal history only Leo and Gregory have been remembered with the sobriquet "the Great."
Readers should begin with the sections on Leo by Karl Baus and others in The Imperial Church from Constantine to the Early Middle Ages, "History of the Church," no. 2 (New York, 1980), pp. 264–269, with a bibliography. The best guide to Leo's letters is still Regesta pontificum Romanorum, vol. 1, 2d ed., edited by Phillip Jaffé (Leipzig, 1885). The letters are usually cited by number preceded by J(affé) K(altenbrunner); an English translation of Leo's letters and sermons by Charles Lett Feltoe is in "A Select Library of Nicene and Post-Nicene Fathers," 2d series, vol. 12 (New York, 1895). Useful still is the informative article by G. N. Bonwetsch in The New Schaff-Herzog Encyclopedia of Religious Knowledge, 13 vols., edited by Samuel M. Jackson (Grand Rapids, Mich., 1953). See also Henry Chadwick's The Early Church (Harmondsworth, 1967) and The Church in Ancient Society (Oxford, 2001).
Robert Somerville (1987 and 2005)