II. The new character of the birth of Christ explained
Therefore, when the time came, dearly beloved, which had been fore-ordained for men's redemption, there enters these lower parts of the world, the Son of God, descending from His heavenly throne and yet not quitting His Father's glory, begotten in a new order, by a new nativity. In a new order, because being invisible in His own nature He became visible in ours, and He whom nothing could contain, was content to be contained: abiding before all time He began to be in time: the Lord of all things, He obscured His immeasurable majesty and took on Him the form of a servant: being God, that cannot suffer, He did not disdain to be man that can, and immortal as He is, to subject Himself to the laws of death. And by a new nativity He was begotten, conceived by a Virgin, born of a Virgin, without paternal desire, without injury to the mother's chastity: because such a birth as knew no taint of human flesh, became One who was to be the Saviour of men, while it possessed in itself the nature of human substance.
For when God was born in the flesh, God Himself was the Father, as the archangel witnessed to the Blessed Virgin Mary: “because the Holy Spirit shall come upon you, and the power of the most High shall overshadow you: and therefore, that which shall be born of you shall be called holy, the Son of God.” The origin is different but the nature like: not by intercourse with man but by the power of God was it brought about: for a Virgin conceived, a Virgin bare, and a Virgin she remained.
Consider here not the condition of her that bare but the will of Him that was born; for He was born Man as He willed and was able. If you inquire into the truth of His nature, you must acknowledge the matter to be human: if you search for the mode of His birth, you must confess the power to be of God. For the Lord Jesus Christ came to do away with not to endure our pollutions: not to succumb to our faults but to heal them. He came that He might cure every weakness of our corruptness and all the sores of our defiled souls: for which reason it behooved Him to be born by a new order, who brought to men's bodies the new gift of unsullied purity.
For the uncorrupt nature of Him that was born had to guard the primal virginity of the Mother, and the infused power of the Divine Spirit had to preserve in spotlessness and holiness that sanctuary which He had chosen for Himself: that Spirit (I say) who had determined to raise the fallen, to restore the broken, and by overcoming the allurements of the flesh to bestow on us in abundant measure the power of chastity: in order that the virginity which in others cannot be retained in child-bearing, might be attained by them at their second birth.
Source: Sermons (New Advent)