Today we celebrate the sanctity of one of the greatest pontiffs to have honored the Roman See. His pontificate lasted for more than 21 years and has been without any doubt one of the most important in the history of the Church. The times in which Pope Leo lived were very difficult: constant barbarian invasions, the gradual weakening of imperial authority in the West and the long, drawn-out social crisis forced the Bishop of Rome to play an important role in civil and political events.

We are familiar with Pope Leo's action thanks to his most beautiful sermons and thanks to his letters. Leo the Great was a tireless champion and upholder of the Roman Primacy, presenting himself as the Apostle Peter's authentic heir: the many Bishops who gathered at the Council of Chalcedon, the majority of whom came from the East, were well aware of this. This Council affirmed the union in the one Person of the Son of God of his two natures, human and divine, without confusion and without separation.

—The Pope asserted this faith in Jesus Christ, true God and true man, in an important doctrinal text addressed to the Bishop of Constantinople, the so-called “Tome to Flavian” which, read at Chalcedon, was received by the Bishops present with an eloquent acclamation. Information on it has been preserved in the proceedings of the Council: "Peter has spoken through the mouth of Leo", the Council Fathers announced in unison.