Pope Leo I ( 400 – 10 November 461), also known as Leo the Great, was bishop of Rome from 29 September 440 until his death. Pope Benedict XVI said that Leo's papacy "was undoubtedly one of the most important in the Church's history." Leo was a Roman aristocrat, and was the first pope to have been called "the Great". He is perhaps best known for having met Attila the Hun in 452 and persuaded him to turn back from his invasion of Italy. He is also a Doctor of the Church, most remembered theologically for issuing the Tome of Leo, a document which was a major foundation to the debates of the Council of Chalcedon, the fourth ecumenical council. That meeting dealt primarily with Christology and elucidated the orthodox definition of Christ's being as the hypostatic union of two natures, divine and human, united in one person, "with neither confusion nor division". It was followed by a major schism associated with Monophysitism, Miaphysitism and Dyophysitism. He also contributed significantly to developing ideas of papal authority. Early life According to the Liber Pontificalis, he was a native of Tuscany. By 431, as a deacon, he was sufficiently well known outside of Rome that John Cassian dedicated to him the treatise against Nestorius written at Leo's suggestion. About this time Cyril of Alexandria appealed to Rome regarding a jurisdictional dispute with Juvenal of Jerusalem, but it is not entirely clear whether the letter was intended for Leo, in his capacity of archdeacon, or for Pope Celestine I directly. Near the end of the reign of Pope Sixtus III, Leo was dispatched at the request of Emperor Valentinian III to settle a dispute between Aëtius, one of Gaul's chief military commanders, and the chief magistrate Albinus. Johann Peter Kirsch sees this commission as a proof of the confidence placed in the able deacon by the Imperial Court. Papacy During Leo's absence in Gaul, Pope Sixtus III died (11 August 440), and on 29 September he was unanimously elected by the people to succeed him. Soon after assuming the papal throne Leo learned that in Aquileia, Pelagians were received into church communion without formal repudiation of their errors; he censured this practice and directed that a provincial synod be held where such former Pelagians be required make an unequivocal abjuration. Manichaeans fleeing the Vandals had come to Rome in 439 and secretly organized