Leo the Great turned Attila back and defined Chalcedon's Christ
Leo I, Bishop of Rome from 29 September 440 to 10 November 461, grounded papal authority in succession from Peter. In 452 he met Attila and persuaded the Hun to spare further Italian invasion. His Tome at Chalcedon framed Christ's two natures in one person, earning him Doctor of the Church.
A Tuscan aristocrat's son according to the Liber Pontificalis, he was already a noted deacon by 431 when John Cassian dedicated an anti-Nestorian treatise at his urging and Cyril of Alexandria sought Roman support. Near Sixtus III's end, Emperor Valentinian III sent Leo to mediate between general Aëtius and magistrate Albinus in Gaul—proof of secular trust.
Chalcedon, the fourth ecumenical council, elevated his Christology across the imperial church. Meeting Attila burnished a pastoral-diplomatic legend that later ages retold as proof of papal soft power amid crumbling western defenses.
Theology plus crisis negotiation made Leo Great: he argued Rome's primacy from Peter while bargaining with armies that no longer answered to a strong western emperor.
Sixtus III died on 11 August 440 while Leo was still in Gaul, and on 29 September the people of Rome chose him unanimously. He moved hard against dissent: in late 443 he preached against Manichaeans he believed were organising secretly in the city, and by 447 he held that heretics deserved the severest penalties, death included. He also pushed Roman practice on other churches, telling Dioscorus of Alexandria in 445 to follow Rome because Mark had been Peter's disciple, and scolding Sicilian bishops over when to baptise. At home he boosted charity for a city strained by famine, refugees and poverty, tying fasting to almsgiving. His Tome, a letter to Flavian of Constantinople, was delivered at Ephesus in 449 and again at Chalcedon in 451, which affirmed Christ's human and divine natures united in one person. Nearly 100 sermons and 150 letters survive.
Source: Pope Leo I
