“It is evident that there are important reasons for ascribing the actio unitiva to the Holy Ghost, and that is why this ascribing has a rich and deep meaning. We shall here summarize the most important reasons. The fact that the actio unitiva is ascribed to the Holy Ghost as its principle characterizes it as an action differing from that of creatures and from that of God Himself in the natural order as much as from the intrinsic action of God proper to His nature. Thus the actio unitiva appears in the first instance as an action of God, and indeed of a supernatural character, the aim of which arises above the natural order; in the second instance, as an actio which proceeds from God as an influence exerted because of a free and infinite love. The accomplishment of the union by the Holy Ghost, who proceeds from the very person who assumes humanity, shows that this person is absolutely complete in Himself and assumes the flesh, not because anything lacking to Him, but by virtue of the fullness of life and power, which reveals itself in the Holy Ghost. As the Holy Ghost is produced last of the persons in the Blessed Trinity, it is likewise He who appears as the natural author of the relations of God ad extra. And as He forms the crowning tie between the Father and the Son, so He effects the tie between the Son and His created nature.
The incarnation of the eternal Word considered as the actual ‘becoming flesh’ presents a further analogy with the embodiment of the interior word from the spirit of man into the spoken word of the mouth. As the spoken word becomes related to the interior word through the breath, which brings forth, so the flesh of the eternal Word is formed by this Word and is united with it through the breath of God. In its full essence the achieving of the union is an infusion and inbreathing of a principle of life by God, analogous to the natural and supernatural completion of the first man: an infusion and inbreathing into human nature, of the eternal Word according to His own subsistence. As such it appears most distinctly when represented as being effected through the effusion of God’s eternal breath of life, and this in a much deeper sense than the completion of the first man.
Lastly, we must understand the Incarnation as the highest form of God’s communication of Himself to the creature, and therefore of the deification, of the endowment with grace, and of the sanctification of the creature. All these effects now are such that they are naturally ascribed to the Holy Ghost and through that fact are strikingly elucidated in their essence.”
-Matthias Scheeben in Mariology, vol. 1