According to the Liber pontificalis he was a native of Tuscany.
By 431, as a deacon, he occupied a sufficiently important position for Cyril of Alexandria to apply to him in order that Rome's influence should be thrown against the claims of Juvenal of Jerusalem[?] to patriarchal jurisdiction over Palestine -- unless this letter is addressed rather to Pope Celestine I. About the same time Johannes Cassianus[?] dedicated to him the treatise against Nestorius written at his request. But nothing shows more plainly the confidence felt in him than his being chosen by the emperor to settle the dispute between Aetius and Albinus, the two highest officials in Gaul.
During his absence on this mission, Pope Sixtus III died (August 11, 440), and Leo was unanimously elected by the people to succeed him. On September 29 he entered upon a pontificate which was to be epoch-making for the centralization of the government of the Church.
An uncompromising foe of heresy, Leo found that in the diocese of Aquileia, Pelagians were received into church communion without formal repudiation of their errors; he wrote to rebuke this culpable negligence, and required a solemn abjuration before a synod.
Manicheans fleeing before the Vandals had come to Rome in 439 and secretly organized there; Leo became aware of this and proceeded against them (c. 443), holding a public debate with their representatives, burning their books, and warning the Roman Christians against them. The edict of Valentinian III against them (June 19, 445) was brought about by his efforts.
Nor was his attitude less decided against the Priscillianists. Bishop Turrubius of Astorga, astonished at the spread of this sect in Spain, had addressed the other Spanish bishops on the subject, sending a copy of his letter to Leo, who did not let slip the opportunity to exercise influence in Spain. He wrote an extended treatise (July 21, 447) against the sect, examining its false teaching in detail, and calling for a Spanish general council to investigate whether it had any adherents in the episcopate -- but this was prevented by the political circumstances of Spain.
Leo enforced his authority in 445 against Dioscurus[?], Cyril's successor in the patriarchate of Alexandria, insisting that the ecclesiastical practise of his see should follow that of Rome; since Mark, the disciple of Peter and founder of the Alexandrian Church, could have had no other tradition than that of the prince of the apostles.
The fact that the African province of Mauretania Caesariensis had been preserved to the empire and thus to the Nicene faith in the Vandal invasion, and in its isolation was disposed to rest on outside support, gave Leo an opportunity to assert his authority there, which he did decisively in regard to a number of questions of discipline.
In a letter to the bishops of Campania, Picenum, and Tuscany (443) he required the observance of all his precepts and those of his predecessors; and he sharply rebuked the bishops of Sicily (447) for their deviation from the Roman custom as to the time of baptism, requiring them to send delegates to the Roman synod to learn the proper practise.
The assertion of Roman power over Illyria had been a strong point with previous popes. Pope Innocent I had constituted the metropolitan of Thessalonica his vicar, in order to oppose the growing power of the patriarch of Constantinople there. But now the Illyrian bishops showed a tendency to side with Constantinople, and the popes had difficulty in maintaining their authority. In 444 Leo laid down in a letter to them the principle that Peter had received the primacy and oversight of the whole Church as a requital of his faith, and that thus all important matters were to be referred to and decided by Rome. In 446 he had occasion twice to interfere in the affairs of Illyria, and in the same spirit spoke of the Roman pontiff as the apex of the hierarchy of bishops, metropolitans, and primates. From the end of the fifth century, however, the influence of Constantinople was again predominant here.
Not without serious opposition did he succeed in asserting his authority over Gaul. Patroclus of Arles (d. 426) had received from Pope Zosimus the recognition of a primacy over the Gallican Church which was strongly asserted by his successor Hilary (429-449). An appeal from Celidonius of Besançon gave Leo occasion to proceed against Hilary, who defended himself stoutly at Rome, refusing to recognize Leo's judicial status. But Leo restored Celidonius and restricted Hilary to his own diocese, depriving him even of his metropolitan rights over the province of Vienne.
Feeling that his dominant idea of the Roman universal monarchy was threatened, Leo appealed to the civil power for support, and obtained from Valentinian III the famous decree of June 6, 445, which recognized the primacy of the bishop of Rome based on the merits of Peter, the dignity of the city, and the Nicene Creed (in their interpolated form); ordained that any opposition to his rulings, which were to have the force of law, should be treated as treason; and provided for the forcible extradition by provincial governors of anyone who refused to answer a summons to Rome. Hilary made his submission, although under his successor, Ravennius, Leo divided the metropolitan rights between Arles and Vienne (450).
A favorable occasion for extending the authority of Rome in the East offered in the renewal of the Christological controversy by Eutyches, who in the beginning of the conflict appealed to Leo and took refuge with him on his condemnation by Flavian. But on receiving full information from Flavian; Leo took his side decisively.
At the "Robber Synod" of Ephesus Leo's representatives delivered the famous "tome" or statement of the faith of the Roman Church in the form of a letter addressed to Flavian, which repeats, in close adherence to Augustine, the formulas of western Christology, without really touching the problem that was agitating the East. The council did not read the letter, and paid no attention to the protests of Leo's legates, but deposed Flavian and Eusebius, who appealed to Rome.
Leo demanded of the emperor that an ecumenical council should be held in Italy, and in the meantime, at a Roman synod in October 449, repudiated all the decisions of the "Robber Synod." Without going into a critical examination of its dogmatic decrees, in his letters to the emperor and others he demanded the deposition of Eutyches as a Manichean and Docetic heretic.
With the death of Theodosius II (450) and the sudden change in the Eastern situation, Anatolius the new patriarch of Constantinople fulfilled Leo's requirements, and his "tome" was everywhere read and recognized.
He was now no longer desirous of having a council, especially since it would not be held in Italy. It was called to meet at Nicaea, then transferred to Chalcedon, where his legates held at least an honorary presidency, and where the bishops recognized him as the interpreter of the voice of Peter and as the head of their body, requesting of him the confirmation of their decrees. He firmly declined to confirm their disciplinary arrangements, which seemed to allow Constantinople a practically equal authority with Rome and regarded the civil importance of a city as a determining factor in its ecclesiastical position; but he strongly supported its dogmatic decrees, especially when, after the accession of the Emperor Leo I (457) there seemed to be a disposition toward compromise with the Eutychians. He succeeded in having an orthodox patriarch, and not the Monophysite Timotheus Ælurus, chosen as patriarch of Alexandria on the murder of Proterius.
The approaching collapse of the Western Empire gave Leo a further opportunity to appear as the representative of lawful authority. When Attila invaded Italy in 452 and threatened Rome, it was Leo who, with two high civil functionaries, went to meet him, and so impressed him that he withdrew -- at least according to Prosper, although Jordanis, who represents Leo's contemporary Priscus, gives other grounds. His intercession could not prevent the sack of the city by Genseric in 455, but murder and arson were repressed by his influence. He died probably on November 10, 461.
The significance of Leo's pontificate lies in the fact of his assertion of the universal episcopate of the Roman bishop, which comes out in his letters, and still more in his ninety-six extant orations.
According to him the Church is built upon Peter, in pursuance of the promise of Matt. xvi. 16-19. Peter participates in everything which is Christ's; what the other apostles have in common with him they have through him. The Lord prays for Peter alone when danger threatens all the apostles, because his firmness will strengthen the others. What is true of Peter is true also of his successors. Every other bishop is charged with the care of his own special flock, the Roman with that of the whole Church. Other bishops are only his assistants in this great task.
Through the see of Peter, Rome has become the capital of the world in a wider sense than before. For this reason, when the earth was divided among the apostles, Rome was reserved to Peter, that here, at the very center, the decisive triumph might be won over the earthly wisdom of philosophy and the power of the demons; and thus from the head the light of truth streams out through the whole body.
In Leo's eyes the decrees of the Council of Chalcedon acquired their validity from his confirmation. The wide range of this theory justifies the application to him of the title of the first pope.
The Roman Catholic church marks 11 April as the feast day of Saint Leo.
The Eastern Orthodox churches mark 18 February as the feast day of Saint Leo.
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