Gregory the Great
Catena Aurea · Lukasevangelium 1
Then said Mary unto the angel, How shall this be, seeing I know not a man? And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
(18 Moral. c. 52. super Job 28:19.) To distinguish His holiness from ours, Jesus is stated in an especial manner to be born holy. For we although indeed made holy, are not born so, for we are constrained by the very condition of our corruptible nature to cry out with the Prophet, Behold, I was conceived in iniquity. (Ps. 51:5.) But He alone is in truth holy, who was not conceived by the cementing of a fleshly union, nor as the heretics rave, one person in His human nature, another in His divine; not conceived and brought forth a mere man, and afterwards by his merits, obtained that He should be God, but the Angel announcing and the Spirit coming, first the Word in the womb, afterwards within the womb the Word made flesh. Whence it follows, Shall be called the Son of God.