The Enchiridion 40
40 Christ’s Birth Through the Holy Spirit Manifests to Us the Grace of God PL 40, 252
And, therefore, as one thing may be born of another, and yet not in such a way as to be its son, and as not every one who is called a son was born of him whose son he is called, it is clear that this arrangement by which Christ was born of the Holy Spirit, but not as His son, and of the Virgin Mary as her son, is intended as a manifestation of the grace of God. For it was by this grace that a man, without any antecedent merit, was at the very commencement of His existence as man, so united in one person with the Word of God, that the very person who was Son of man was at the same time Son of God, and the very person who was Son of God was at the same time Son of man; and in the adoption of His human nature into the divine, the grace itself became in a way so natural to the man, as to leave no room for the entrance of sin. Wherefore this grace is signified by the Holy Spirit; for He, though in His own nature God, may also be called the gift of God. And to explain all this sufficiently, if indeed it could be done at all, would require a very lengthened discussion.
Modus quo natus est Christus de Spiritu sancto, insinuat gratiam unionis hypostaticae. Cum itaque de aliquo nascatur aliquid etiam non eo modo ut sit filius, nec rursus omnis qui dicitur filius, de illo sit natus cujus dicitur filius; profecto modus iste quo natus est Christus de Spiritu sancto non sicut filius, et de Maria virgine sicut filius, insinuat nobis gratiam Dei, qua homo nullis praecedentibus meritis, in ipso exordio naturae suae quo esse coepit, Verbo Deo copularetur in tantam personae unitatem, ut idem ipse esset filius Dei qui filius hominis, et filius hominis qui filius Dei: ac sic in naturae humanae susceptione fieret quodam modo ipsa gratia illi homini naturalis, quae nullum peccatum posset admittere. Quae gratia propterea per Spiritum sanctum fuerat significanda, quia ipse proprie sic est Deus, ut dicatur etiam Dei donum . Unde sufficienter loqui (si tamen id fieri potest), valde prolixae disputationis est.