St. Leo the Great
Today the Church celebrates the memorial of St. Leo the Great, pope and doctor, 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. His feastday in the Extraordinary Rite is April 11.