He was engendered by a new kind of birth, conceived by a Virgin, born of a Virgin, without a father’s carnal concupiscence, without injuring his Mother’s integrity. Indeed, such a birth was appropriate for the future Savior of men, Who, while sharing the nature of human substance, did not know the contamination of human flesh. The Author of God taking flesh is God himself, as the archangel witnesses to the Blessed Virgin Mary: “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; hence, the holy offspring born of you will be called Son of God” (Luke 1:35). In His origin unlike us; in His [human] nature like us — our common human customs are of no help here. It was decided by God’s almighty power that Mary should conceive as a virgin, give birth as a virgin, and remain a virgin. Do not think about the condition of his Mother, but consider the decision of the Son, who wanted in this way to be born a man and so brought it about. If you seek the truth about His nature, then recognize the matter as human; if you want to find the secret of His origin, then acknowledge the divine power. For our Lord Jesus Christ came to take away our infection, not to be infected by it; He did not come to succumb to our vices but to heal them. He came to heal the malady of our corruption and all the wounds of our scarred souls. For this reason He had to be born in a new manner, since He was bringing the new grace of spotless integrity to our human bodies. He had to keep His Mother’s original virginity intact, and it was necessary that the power of the Holy Spirit should safeguard the defense of her modesty, which He was pleased to call the dwelling place of holiness.
For He had decided to raise up what was fallen and restore what was broken apart and to strengthen purity for overcoming the seductions of the flesh, so that virginity, which in others cannot be preserved after childbirth, might be imitated by others, in rebirth.
—Pope St. Leo the Great
Sermo 22, 2 (Migne, Patrologia Latina 54, 195-196)
in Luigi Gambero, Mary and the Fathers of the Church
(San Francisco: Ignatius, 1991), 309