From Sermon 22, On the Feast of the Nativity (ca. 440-461).

For God the almighty and merciful…foretold at the very beginning of the world the remedy…proclaiming to the serpent that the seed of the woman should come to crush the lifting of his baneful head by its power, signifying no doubt that Christ would come in the flesh, God and man, who, born of a Virgin, should by his uncorrupt birth condemn the despoiler of the human stock. Thus in the whole and perfect nature of true man was true God born, complete in what was His own, complete in what was ours.

And ours we call what the Creator formed in us from the beginning and what he undertook to repair. For what the deceiver brought in and the deceived admitted had no trace in the Savior. Nor because He partook of man’s weaknesses, did he therefore share our faults. He took the form of a slave without stain of sin, increasing the human and not diminishing the Divine: because that emptying of himself whereby the Invisible made himself visible and Creator and Lord of all things as he was, wished to be mortal, was the condescension of pity not the failing of power.

Therefore, when the time came, dearly beloved, which had been fore-ordained for men’s redemption, there enters these lower parts of the world, the Son of God, descending from his heavenly throne… He whom nothing could contain, was content to be contained: abiding before all time He began to be in time. The Lord of all things, He obscured His immeasurable majesty and took on Him the form of a servant: being God, that cannot suffer, He did not disdain to be subject himself to the laws of death. And by a new nativity he was begotten, conceived by a Virgin… For when God was born in the flesh, God Himself was the Father, as the archangel witnessed to the Blessed Virgin Mary: because the Holy Spirit shall come upon you, and the power of the most high shall overshadow you: and therefore, that which shall be born of you shall be called holy, the Son of God (Luke 1:35) .

When, therefore, the merciful and almighty Savior began His human course, hiding the power of his Godhead which was inseparable from his manhood under the veil of our weakness, the crafty foe – the devil – was taken off his guard and he thought that the nativity of the child, who was born for the salvation of mankind, was as much subject to himself as all others are at their birth. For he saw him crying and weeping, he saw him wrapped in swaddling clothes, subjected to circumcision, offering the sacrifice which the law required. And then he perceived in Christ the usual growth of boyhood and could have had no doubt of his reaching man’s estate by natural steps.

Then, the devil inflicted insults, multiplied injuries, made use of curses, affronts, blasphemies, abuse, in a word, poured upon him all the force of his fury and exhausted all the varieties of trial: and knowing how he had poisoned man’s nature, had no idea that Christ had no share in the first transgression whose mortality he had ascertained by so many proofs. The unscrupulous thief and greedy robber persisted in assaulting him who had nothing of his own, and in carrying out the general sentence on original sin, went beyond the bond on which he rested, and required the punishment of iniquity from him in whom he found no fault.

And thus the malevolent terms of the deadly compact are annulled, and through the injustice of an overcharge the whole debt is cancelled. The strong one is bound by his own chains, and every device of the evil one recoils on his own head. When the prince of the world is bound, all that he held in captivity is released. Our nature cleansed from its old sickness regains its honorable estate, death is destroyed by death, nativity is restored by nativity: since at one and the same time redemption does away with slavery, regeneration changes our origin, and faith justifies the sinner.

Now, estimate this atonement at its right worth. For to you who were castaways, banished from the realms of paradise, dying of your weary exile, reduced to dust and ashes, without further hope of living, by the incarnation of the Word was given the power to return from afar to your Maker… to become free after slavery, to be promoted from being an outcast to sonship: so that, you who were born of corruptible flesh, may be reborn by the Spirit of God, and obtain through grace what you had not by nature, and, if you acknowledge yourself the son of God by the spirit of adoption, dare to call God Father. Freed from the accusings of a bad conscience, aspire to the kingdom of heaven, do God’s will supported by the Divine help, imitate the angels upon earth, feed on the strength of immortal sustenance, fight fearlessly on the side of holiness against hostile temptations, and if you keep your allegiance in the heavenly warfare, doubt not that you will be crowned for your victory in the triumphant camp of the Eternal King, when the resurrection that is prepared for the faithful has raised you to participate in the heavenly kingdom.

Having therefore so confident a hope, dearly beloved, abide firm in the faith in which you are built: lest that same tempter whose tyranny over you Christ has already destroyed, win you back again with any of his wiles, and mar even the joys of the present festival by his deceitful art……Honor the sacred and divine mystery of human restoration with holy and sincere service. Embrace Christ born in our flesh, that you may deserve to see him also as the God of glory reigning in his majesty, who with the Father and the Holy Spirit remains in the unity of the Godhead for ever and ever. Amen.

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.