II. The Virgin's conception explained
In celebrating therefore the birthday of our Lord and Saviour, dearly beloved, let us entertain pure thoughts of the blessed Virgin's child-bearing, so as to believe that at no moment of time was the power of the Word wanting to the flesh and soul which she conceived, and that the temple of Christ's body did not previously receive its form and soul that its Inhabitant might come and take possession but through Himself and in Himself was the beginning given to the New Man, so that in the one Son of God and Man there might be Godhead without a mother, and Manhood without a Father.
For her virginity fecundated by the Holy Spirit at one and the same time brought forth without trace of corruption both the offspring and the Maker of her race. Hence also the same Lord, as the Evangelist relates, asked of the Jews whose son they had learned Christ to be on the authority of the Scriptures, and when they replied that the tradition was He would come of David's seed, “How,” says He, “does David in the Spirit call Him Lord, saying, the Lord said to my Lord: sit on My right hand till I place your enemies as the footstool of your feet?” And the Jews could not solve the question put, because they did not understand that in the one Christ both the stock of David and the Divine nature were there prophesied.
Source: Sermons (New Advent)