“Why is it hard to believe that Mary gave birth in a way contrary to the law of natural birth and remained a virgin, when contrary to the law of nature the sea looked at Him and fled, and the waters of the Jordan returned to their source (Ps. 113:3). Is it past belief that a virgin gave birth when we read that a rock issued water (Ex. 17:6), and the waves of the sea were made solid as a wall (Ex. 14:22)? Is it past belief that a Man came from a virgin when a rock bubbled forth a flowing stream (Ex. 20:11), iron floated on water (4 Kings 6:6), a Man walked upon the waters (Mt. 14:26)? If the waters bore a Man, could not a virgin give birth to a man? What Man? Him of Whom we read: ‘…the Lord shall be known to the Egyptians, and the Egyptians shall know the Lord in that day; and they shall offer sacrifices, and shall vow vows to the Lord, and pay them’ (Is. 19:20).
+ St. Ambrose, Synodal Letter 44, Letters, 1-91 (Fathers of the Church Patristic Series)