CHAPTER XI. The Incarnation as Prime Example of the Action of God's Grace
36. In this the grace of God is supremely manifest, commended in grand and visible fashion; for what had the human nature in the man Christ merited, that it, and no other, should be assumed into the unity of the Person of the only Son of God? What good will, what zealous strivings, what good works preceded this assumption by which that particular man deserved to become one Person with God? Was he a man before the union, and was this singular grace given him as to one particularly deserving before God? Of course not! For, from the moment he began to be a man, that man began to be nothing other than God's Son, the only Son, and this because the Word of God assuming him became flesh, yet still assuredly remained God. Just as every man is a personal unity-that is, a unity of rational soul and flesh-so also is Christ a personal unity: Word and man.
Why should there be such great glory to a human nature-and this undoubtedly an act of grace, no merit preceding unless it be that those who consider such a question faithfully and soberly might have here a clear manifestation of God's great and sole grace, and this in order that they might understand how they themselves are justified from their sins by the selfsame grace which made it so that the man Christ had no power to sin? Thus indeed the angel hailed his mother when announcing to her the future birth: "Hail," he said, "full of grace." And shortly thereafter, "You have found favor with God."  And this was said of her, that she was full of grace, since she was to be mother of her Lord, indeed the Lord of all. Yet, concerning Christ himself, when the Evangelist John said, "And the Word became flesh and dwelt among us," he added, "and we beheld his glory, a glory as of the only Son of the Father, full of grace and truth."  When he said, "The Word was made flesh," this means, "Full of grace." When he also said, "The glory of the only begotten of the Father," this means, "Full of truth." Indeed it was Truth himself, God's only begotten Son-and, again, this not by grace but by nature-who, by grace, assumed human nature into such a personal unity that he himself became the Son of Man as well.
37. This same Jesus Christ, God's one and only Son our Lord, was born of the Holy Spirit and the Virgin Mary. Now obviously the Holy Spirit is God's gift, a gift that is itself equal to the Giver; wherefore the Holy Spirit is God also, not inferior to the Father and the Son. Now what does this mean, that Christ's birth in respect to his human nature was of the Holy Spirit, save that this was itself also a work of grace?
For when the Virgin asked of the angel the manner by which what he announced would come to pass (since she had known no man), the angel answered: "The Holy Spirit shall come upon you and the power of the Most High shall overshadow you; therefore the Holy One which shall be born of you shall be called the Son of God."  And when Joseph wished to put her away, suspecting adultery (since he knew she was not pregnant by him), he received a similar answer from the angel: "Do not fear to take Mary as your wife; for that which is conceived in her is of the Holy Spirit"  -that is, "What you suspect is from another man is of the Holy Spirit."