hough born to a Tuscan family, he was in Rome at a young age. Nothing certain is known of his upbringing, except at some point he chose an ecclesiastical career. By 430 he was advising Pope Celestine and was pressing John Cassian to write against Nestorius. While L. was engaged in a diplomatic mission to Gaul, he was chosen new Bishop of Rome and was ordained in 440. He aggressively attacked heretical movements, especially the Manichees against whom he persuaded Valentinian III to reinstate laws. His greatest controversy was occasioned by Eutyches, an archimandrite, who argued that Christ had only one nature (Monophysite position). L. responded to Flavian, bishop of Constantinople, with the famous Tomus ad Flavianum(Ep. 28), which laid down the western doctrine of the two natures of Christ: una persona in duabus naturis. A synod in 449, which L. later condemned as the "robber synod," briefly restored Eutyches. The Council of Chalcedon, summoned by the Emperor Marcian in 450, convened in 451 with L. at its head as the Bishop of Rome and condemned Eutyches on the basis of the Tomus and Cyril's Second Letter to Nestorius and his letter to John of Antioch containing the Formula of Reunion. The Council also ruled that the two natures come together in Christ "unconfusedly, unalterably, undividedly and inseparably." L. argued for the primacy of the Roman bishop because he claimed the position was in direct line to Peter, whom Christ specifically chose as the one upon whom he would build his church. The church's hierarchy, for him, had "the shape of a pyramid with the bishop of Rome at the top" (Drobner 479). L. also saved Rome twice from being sacked: once from the Huns led by Attila in 452 and once from the Vandals in 455. L. died in 461, having theoretically established orthodox Christology and the primacy of Rome while conflicts concerning Christology raged on.

L. left behind 173 letters, 30 of which are addressed to him, with numbers 23 and 120 perhaps not being genuine. In his Christology and much of his theology, he reveals heavy influence from Augustine. However, in light of his whole theology, it would be accurate to say he added to Augustine's thought a rich understanding of the Incarnation and the subsequent presence of God, especially in the liturgy. In addition to the letters, we also have 97 sermons of L.'s, a significant number of which follow the liturgical year and which bring out this theme of the "here-ness" of God's acts of salvation. Because scholars are able to date the sermons accurately, they provide a valuable window into liturgical practices of the Church and an understanding of those practices.

L. rejected canon 28 of the Council of Chalcedon, which claimed Constantinople to be exalted above Alexandria and Antioch and essentially equal to Rome in authority. Naturally, this was a cause of controversy leading up to and following the split of the eastern and western Churches. He was the first pope to earn the name "the Great" because of the widespread recognition that in his person were combined superior administrative skill with a first-class theological mind.