400 – 10 November 461), also known as Saint Leo the Great, was Bishop of Rome from 29 September 440 and died in 461.
Pope Benedict XVI said that Leo's papacy "...was undoubtedly one of the most important in the Church's history."He 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 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 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.