Hierarch Saint Leo the Great, Pope of Rome

Saint Leo the Great, Pope of Rome (born c. 390–400, died 461), was among the foremost Church Fathers of the 5th century, a theologian and defender of Orthodox Christology. Elected Bishop of Rome in September 440.

He is best known for his doctrinal letter to Patriarch Flavian of Constantinople — the famous "Tome of Leo" — which clearly articulated the union of Christ's divine and human natures "without confusion and without separation." Read aloud at the Fourth Ecumenical Council of Chalcedon in 451, the Tome became foundational to the Chalcedonian definition of faith; the assembled bishops cried out, "Peter has spoken through Leo!"

Leo is also remembered as a defender of Rome itself: in 452 he led a delegation that persuaded Attila the Hun to withdraw from Italy without sacking the city, and in 455 he similarly moderated the Vandal king Geiseric's assault on Rome. He left behind roughly 96 sermons and 143 letters. He reposed on November 10, 461, and is buried in St. Peter's Basilica.