Pope Saint Leo I (400 –461), also known as Saint Leo the Great, was pope from 440 to his death in 461. He was the first pope to have been called “the Great.” He is perhaps best known for having met Attila the Hun in 452, who had set out to conquer all of Europe and persuading 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 Ecumenical Council of Chalcedon. The Council of Chalcedon, the fourth ecumenical council, dealt primarily with Christology, and elucidated the definition of Christ’s being the hypostatic union of two natures, divine and human, united in one per-son, “with neither confusion nor division.” He worked hard to dispel the prominent heresies of his day; Pelagianism (overemphasizing human freedom,) Manichaeism (seeing everything material as evil.) Saint Leo held firmly that everything he did and said as pope for the Church represented Christ, the head of the Mystical Body, and Saint Peter, in whose place Leo acted.