In the very act in which we are reverencing the birth of our Savior, we are also celebrating our own new birth. For the birth of Christ is the origin of the Christian people; and the birthday of the head is also the birthday of the body [the Church]. As the whole community of the faithful, once begotten in the baptismal font, was crucified with Christ in the passion, raised up with him in the resurrection, and at the ascension placed at the right hand of the Father, so too it is born with him in this nativity.

For all believers regenerated in Christ, no matter in what part of the whole world they may be, break with that ancient way of life that derives from original sin, and by rebirth are transformed into new persons. Henceforth they are reckoned to be of the stock, not of their earthly father, but of Christ, who became Son of Man precisely so that they could become children of God; for unless in humility he had come down to us, none of us by our own merits could ever go up to him.

—Leo the Great, Sermon 6 for the Nativity

Beloved, our Lord Jesus Christ, the eternal creator of all things, today became our Savior by being born of a mother. Of his own will he was born for us today, in time, so that he could lead us to his Father’s eternity. God became human like us so that we might become God.

We sinned and became guilty; God is born as one of us to free us from our guilt. We fell, but God descended; we fell miserably, but God descended mercifully; we fell through pride, God descended with his grace.

—Augustine, Sermon 13 on the Time

Let us announce the coming of our Savior with joy! On Christmas strength took on weakness that weakness might become strong. From his human birth let us come to understand how God emptied himself for our sake.

—Augustine, Sermon 190, 3