From Sermon 51, On the Transfiguration

In this transfiguration the foremost object was to remove the offense of the cross from the disciple’s heart, and to prevent their faith being disturbed by the humiliation of Christ’s voluntary passion by revealing to them the excellence of his hidden dignity. But with no less foresight, the foundation was laid of the holy Church’s hope, that the whole body of Christ might realize the character of the change which it would have to receive, and that the members might promise themselves a share in that honor which had already shone forth in their head…

For Moses and Elijah, that is the law and the prophets, appeared talking with the Lord; that in the presence of those five men might most truly be fulfilled what was said: In two or three witnesses stands every word (Deut. 19:15). What is more stable, what more steadfast than this word, in the proclamation of which the trumpet of the Old and of the New Testament joins, and the documentary evidence of the ancient witnesses combine with the teaching of the Gospel?

For the pages of both covenants corroborate each other, and he whom, under the veil of mysteries, the types that went before had promised, is displayed clearly and conspicuously by the splendor of the present glory.

Because, as says blessed John, the law was given through Moses: but grace and truth came through Jesus Christ in whom is fulfilled both the promise of prophetic figures and the purpose of the legal ordinances (John 1:17). He both teaches the truth of prophecy by his presence and renders the commands possible through grace.

And so, while Christ was yet speaking, behold a bright cloud overshadowed them, and behold a voice out of the cloud, saying, “This is my beloved son, in whom I am well pleased; hear him.” The Father was indeed present in the Son, and in the Lord’s brightness, which he had tempered to the disciples’ sight… This is my son, not adopted, but true born, not created from another source, but begotten of me: nor yet made like me from another nature, but born equal to me of my nature. This is my son, through whom all things were made, and without whom was nothing made because all things that I do he does in like manner: and whatever I perform, he performs with me inseparably and without difference: for the son is in the father and the Father in the Son, and our unity is never divided: and though I am one who begot, and he the other whom I begot, yet is it wrong for you to think anything of him which is not possible of me.

This is my Son, who sought not by grasping, and seized not in greediness, that equality with me which he has. But remaining in the form of my glory, that he might carry out our common plan for the restoration of mankind, he lowered the unchangeable Godhead even to the form of a slave.

St. Leo the Great (ca. 400-461) was a Roman cleric and theologian. He served as a diplomat for the papal court and became Bishop of Rome in 440, exercising pastoral care during the depredations of the Huns and the Vandals. His Tome, a clear defense of the the teaching that the one person of Christ has two natures, divine and human, was adopted by the Council of Chalcedon as a crucial marker of orthodoxy. His feast day is November 10.