The Church celebrates the Memorial of St. Leo the Great (400-461), during whose pontificate the Council of Chalcedon (451) defined that Christ is one divine person with two natures, divine and human. It was a confirmation of his Epistola Dogmatica (Tomus) to the Patriarch Flavian of Constantinople. He vigorously defended the unity of the Church. He detained the onrush of the barbarians under Attila.