In chapter 12 Augustine considers the role of the Holy Spirit in the Incarnation. Though we say that Christ was born of the Holy Spirit and the Virgin Mary, we don’t call him the son of the Spirit. Should we say that his divine nature is the Son of the Father but that his human nature was the Son of the Spirit? No, because that would divide his person.

He goes on to discuss the various senses in which we might say something is born of something else and the different respects in which someone might be called a son (by birth, by adoption). His point is that not everything which is born of something else is called that thing’s son, nor are all sons sons by birth. One wonders whether part of Augustine’s intent here is to counteract attempts to portray Christianity as another pagan myth where the god impregnates a human woman with his offspring.

So, we don’t want to call Jesus the Son of the Spirit, and yet the Spirit plays a special role in his conception and birth. Augustine’s explanation is to connect this to grace:

Wherefore, since a thing may be “born” of something else, yet not in the fashion of a “son,” and conversely, since not everyone who is called son is born of him whose son he is called–this is the very mode in which Christ was “born” of the Holy Spirit (yet not as a son), and of the Virgin Mary as a son–this suggests to us the grace of God by which a certain human person, no merit whatever preceding, at the very outset of his existence, was joined to the Word of God in such a unity of person that the selfsame one who is Son of Man should be Son of God, and the one who is Son of God should be Son of Man. Thus, in his assumption of human nature, grace came to be natural to that nature, allowing no power to sin. This is why grace is signified by the Holy Spirit, because he himself is so perfectly God that he is also called God’s Gift. Still, to speak adequately of this–even if one could–would call for a very long discussion.

Being born of the Holy Spirit indicates, then, that the unity effected between the human and divine natures is from first to last an act of God’s grace. This forecloses both adoptionism and safeguards the full humanity and full divinity of Jesus. Speaking of him as the Son of the Holy Spirit and Mary might indicate some kind of human-divine hybrid. Instead, the orthodox teaching is the human nature was fully united to the divine life, by grace alone, without ceasing to be human.