Leo I, Saint (Saint Leo the Great), c.400–461, pope (440–61), an Italian; successor of St. Sixtus III. A Doctor of the Church, he was one of the greatest pontiffs of the early years of the church. He waged a firm campaign against schism and heresy. With the aid of Valentinian III, the Roman emperor of the West, he campaigned to eliminate Manichaeism from Italy. Later, asserting his authority over St. Hilary of Arles, he obtained an imperial rescript that effectively confirmed the authority of the pope over all his bishops. In the Nestorian-Monophysite controversy Leo was the leader in defending Catholic teaching. He wrote the celebrated Tome of Leo, a doctrinal letter defining the two natures and one person of Christ that was later adopted as ecumenical at Chalcedon (see Chalcedon, Council of), when the heresiarch Eutyches was condemned. He was also effective as a statesman and met (452) Attila the Hun to persuade him not to invade Rome. In 455 he similarly urged Gaiseric the Vandal to spare the lives of the Romans. 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. The celebrated Leonian Sacramentary, the oldest form of the Roman Missal, is probably not his work. He was succeeded by St. Hilary. Feast: Apr. 11.
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