c.400-461. Pope from 440. Born in Tuscany, he was a deacon under Pope (422- 32) and was active before his election as bishop of Rome, succeeding Sixtus III. One of the greatest administrators of the early church, Leo is known for combining Roman Law with ecclesiastical procedure and for strengthening the primacy of the Roman see in church structure.
As pope, Leo was dedicated to the duty of preaching, and he wrote many sermons for the liturgical cycle. He vigorously enforced uniformity in church government and doctrine, both locally (i.e., in the ten surrounding bishoprics) and universally. When the co-emperor Marcian convened thein 451, Leo sent representatives and his Tome to Flavian (patriarch of Constantinople), part of which concerned a doctrine of Christ that was adopted by the council. He maintained peaceful relations with Marcian's successor, Emperor Leo the Thracian, in spite of some friction over the support of Chalcedon. Leo's belief in the use of moderation in the wielding of power is illustrated by his dealings with the African Church and his violent reaction to the discovery that one of his vicars (Anastasius of Thessalonica) had acted hastily in dealing with his assistants. He defended the faith against such heretical groups as the Manichaeans, Monophysites, and Pelagians. He was also instrumental in preventing the destruction of Rome in 452, when he persuaded Attila to withdraw, and in 455, when he persuaded the Vandals to refrain from murdering the populace.
Leo did not write any treatises, but his preserved sermons indicate his beliefs in matters of doctrine. The essence of his teaching lies in his awe for the mystery of Christ and the church. He believed that he acted in the place of Peter, and that Christ actively participated in the governing of the church.
See W.J. Halliwell, The Style of Pope St. Leo the Great (1939), and T.G. Jalland, The Life and Times of St. Leo the Great (1941).