Entrance Antiphon, Cf. Sir 45:30: The Lord established for him a covenant of peace, and made him the prince, that he might have the dignity of the priesthood for ever.Gospel Verse, Mk 1:17: Come after me, says the Lord, and I will make you fishers of men.Communion Antiphon, Mt 16:16,18: Peter said to Jesus: You are the Christ, the Son of the living God. And Jesus replied: You are Peter, and upon this rock I will build my Church.Today the Church celebrates the Memorial of St. Leo the Great, pope and doctor (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.