When the controversy between Cyril of Alexandria and Nestorius erupted in 429–431, it was Leo who, on behalf of Pope Celestine, recruited John Cassian to write against the errors of Nestorius.
Cassian in turn later honored Leo, calling him "the ornament of the Roman church and of the divine ministry". We also have Leo’s own testimony that Cyril of Alexandria wrote to him in 431 in order to gain his support in a controversy concerning Juvenal’s attempt to promote the patriarchate of Jerusalem.
Under Pope Sixtus III (432–440) Leo continued his service as archdeacon, supporting the pope in his efforts against the Pelagian, Julian of Eclanum. While away in Gaul on a diplomatic mission in 440, Leo learned of his election to succeed Sixtus III and returned to Rome for his coronation.
He possessed a very strong conviction that each pope was the direct successor to Peter—and acted as Peter for the sake of the church. This keen conviction of his own role as Peter for the church marked all that he did in the 21 years of his reign.
Leo can be credited with three primary accomplishments. First, he was an effective teacher of the Christian faith. Leo’s short homilies (96 in all) reveal both his oratorical skill and his theological insight. He was not a speculative theologian like Augustine of Hippo, but he had a remarkable ability to synthesize eloquently the essentials of the Christian faith.
His homilies are largely on the feasts of the church year, and they weave together the profound truths of Christian doctrine with a practical orientation to living the Christian life day to day. Leo also carried a particular burden for the poor; his preaching is marked with a consistent plea for alms and for works of charity.
Leo’s second noteworthy accomplishment is as pastor of the Western Latin Church. His letters show how active he was in handling disputes, appointing bishops, and offering pastoral wisdom for many crises of his day. Perhaps his most famous intervention occurred in 452, when he traveled from Rome to Mantua to confront Attila the Hun who was ravaging all of northern Italy.
Somehow Leo persuaded Attila to stop his approach toward Rome and withdraw. A few years later, in 455 he confronted the Vandal leader, Gaiseric, outside Rome’s city walls, and persuaded him to limit his destruction of the city.
Leo is best known, however, for the role he played in shaping the doctrine of Christ. He was a key player in what is known as the Christological controversy of the fifth century. In the midst of a debate in 448 between Flavian and the monk Eutyches, Leo composed a doctrinal letter (known as his Tome) on the two natures of Christ.
Initially rejected, Leo’s Tome played a crucial role in the Council of Chalcedon in 451. Leo is rightly credited with helping to forge the doctrine of the Incarnation for both the Western Latin and the Eastern Greek Church.
He died in 461. Because of his many accomplishments—theological, pastoral, and societal—he earned the title "great" and was declared a Doctor of the Western Church by Pope Benedict XIV in 1754.