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