From the Great Vespers of the Feast at Psalm 140 in tone 2:
Come, let us rejoice in the Lord as we declare this present mystery.
The middle wall of partition is broken asunder; the flaming
sword is turned back, the Cherubim withdraw from the Tree of Life, and
I partake of the Paradise of Delight, whence I was cast out before
through disobedience. For the identical Likeness of the Father, the Express
Image of His eternity, taketh the form of a servant, and without
undergoing change He cometh forth from a Mother that knew not wedlock.
For that which He was, He hath remained, even true God; and that
which He was not, He hath taken upon Himself, becoming man out of
love for man. Unto Him let us cry: O God, Who art born of a Virgin,
have mercy on us.
From a sermon on the Nativity by our father among the saints, St. Peter Chrysologus:
Even if you did not enjoy free access to knowledge of all these marvels, would you think that God was unable at that time to assume from flesh what in the beginning he took from mud? Indeed, since everything is possible to God, and it is impossible for you fully to understand even the least of His works, do not pry too much into this virgin’s conceiving, but believe it. Be reverently aware of the fact that God wishes to be born, because you offer an insult if you examine it too much. Grasp by faith that great mystery of the Lord’s birth, because without faith you cannot comprehend even the least of God’s works. “All his works,” says the Scripture, “are by faith”. But here is a matter which depends completely upon faith, and you want it to stand by reason. It is not, indeed, without reason that this matter holds true; it holds true by the reasoning of God, O man, not yours. What is so much according to reason as the fact that God can do whatever He has willed? He who cannot do what he wills is not God.