He was an Italian aristocrat and the first Pope of the Italian Catholic Church to receive the title "the Great" . He is perhaps best known for having met Attila the Hun outside Italy in 452 in an attempt to persuade the king not to sack the city. He is also a Doctor of the Church, and a leading figure in the centralization of the organization of the Roman Catholic Church.
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 Roman Church.
Manicheans fleeing before the Vandals had come to Rome in 439 and secretly organized there; Leo learned of this around 443, and proceeded against them by holding a public debate with their representatives, burning their books, and warning the Roman Christians against them. His efforts led to the edict of Valentinian III against them (June 19, 445).
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 took the opportunity to exercise Roman policy 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.
In 445, Leo disputed with Pope Dioscorus, St. Cyril's successor as Pope 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. This, of course, was not the position of the Copts, who saw the ancient patraiarchates as equals.
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 practice.
Because of the earlier line of division between the western and eastern parts of the Roman Empire, Illyria was ecclesiastically subject to Rome. Pope Innocent I had constituted the metropolitan of Thessalonica his vicar, in order to oppose the growing influence of the patriarch of Constantinople in the area. In a letter of about 446 to a successor Bishop of Thessalonica, Anastasius, Leo reproached him for the way he had treated one of the metropolitan bishops subject to him; after giving various instructions about the functions entrusted to Anastasius and stressing that certain powers were reserved to the Pope himself, Leo wrote: "The care of the universal Church should converge towards Peter's one seat, and nothing anywhere should be separated from its Head.
In 451 at the Council of Chalcedon, after Leo's Tome on the two natures of Christ was read out, the bishops participating in the Council cried out: "This is the faith of the fathers ... Peter has spoken thus through Leo ...
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 was 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.
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 in 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 imperial patriarch, and not the Oriental Orthodox Pope Timotheus Aelurus, chosen as Coptic Orthodox Pope of Alexandria on the murder of Greek Patriarch Proterius of Alexandria.
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 effected his withdrawal. According to Prosper of Aquitaine, he was so impressed by him that he withdrew. Jordanes, who represents Leo's contemporary Priscus, gives other grounds. Pragmatic concerns such as the large sum of gold that accompanied Leo, or logistical and strategic concerns, may have been the true reason for Attila's mercy. Attila's army was already quite stretched and full from booty from plunder, the Pope's plea for mercy may well have merely served as an honorable reason to not continuing on and sacking the Roman capitol. Other sources of Catholic hagiographical information cite that an enormously huge man dressed in priestly robes and armed with a flaming sword, visible only to Attila, threatened him and his army with death during his discourse with Pope Leo, and this prompted Attila to submit to the Pope's request. Unfortunately Leo's intercession could not prevent the sack of the city by the Vandals in 455, but murder and arson were repressed by his influence. He died probably on November 10, 461.
According to him the Church is built upon Peter, in pursuance of the promise of Matthew 16:16-19. Peter participates in everything which is Christ's; what the other apostles have in common with him they have through him. 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. In Leo's eyes the decrees of the Council of Chalcedon acquired their validity from his confirmation.
St. Leo's letters and sermons reflect the many aspects of his career and personality, including his great personal influence for good, and are invaluable historical sources. His rhythmic prose style, called cursus leonicus, influenced ecclesiastical language for centuries