Patristics

“... 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 that 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.” Augustine, Enchiridion, XL, NPNF1, 3:251