Leo I
Pope from 440 until his death (c. 400–461), known as Leo the Great, and the first bishop of Rome whose theological and political personality is known in sufficient detail through his own writings: about a hundred sermons and more than a hundred letters survive, an exceptional corpus for the period that allows his thought and governance to be reconstructed with precision. He held the papacy at a critical moment, as the Western Roman Empire was collapsing irreversibly under successive barbarian onslaughts, and he is credited, with good historical documentation, with personally persuading Attila the Hun to withdraw from Italy without attacking Rome in 452, in a legendary meeting at the gates of the city near the river Mincio, an episode later tradition and art —including Raphael— magnified with miraculous elements; three years later, in 455, he could not prevent the sack of Rome by Genseric's Vandals, but he managed to negotiate personally that widespread slaughter, the burning of the city and the destruction of its principal churches be avoided. His crucial theological contribution is the Tomus Leonis (Tome of Leo), the doctrinal letter he sent to the Council of Chalcedon in 451, in which he formulates with lasting classical precision the doctrine of Christ's two natures —fully divine and fully human, united 'without confusion, without change, without division, without separation' in a single person— a formula the council accepted, acclaiming 'Peter has spoken through Leo', and which became the orthodox Christological definition still upheld today by most major Christian confessions, including Catholicism, Orthodoxy and many Protestant churches. He also reinforced, with Roman legal and biblical arguments combined with notable rhetorical skill, the doctrine of the primacy of the bishop of Rome over the whole Church as the direct successor of the apostle Peter, thereby laying the theoretical and institutional foundations of the later medieval papacy.