The Enchiridion by St. Augustine
Chapter 40.--Christ's Birth Through the Holy Spirit Manifests to Us the Grace of God.
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.