Pope Leo I (400–461, pope 440–461) was 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. His Feast Day is April 11.