Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter II. He infers from what he has said that the Virgin Mary gave birth to a Son who had pre-existed and was greater than she herself was.

Chapter II.

He infers from what he has said that the Virgin Mary gave birth to a Son who had pre-existed and was greater than she herself was.

You see then that when the Apostle says that God sent His Son, it was His own Son to use the actual words of the Apostle, “His own Son” that God sent. For, since He sent His own Son, it was not some one else’s Son that He sent, nor could He send Him at all if He who was sent had no existence. He sent then, he says, “His own Son, made of a woman.” Therefore because He sent Him, He sent one who existed: and because He sent His own, it certainly was not another’s but His own whom He sent. What then becomes of that argument of yours drawn from this world’s subtleties? No one ever yet gave birth to one who had already existed before. For had not the Lord a pre-existence before Mary? Was not the Son of God existent before the daughter of man? In a word did not God Himself exist before man—since certainly there is no man who is not from God. You see then that I do not merely say that Mary gave birth to one who had existed before her, not only, I say, one who had existed before her, but one who was the author of her being, and that in giving birth to her Creator, she became the mother of Him who gave her being: because it was as simple for God to bring about birth for Himself as for man and as easy for Him to arrange that He Himself should be born of mankind, as that a man should be born. For the power of God is not limited in regard to His own Person, as if what was allowable to Him in the case of all others, was not allowable in His own case, and as if He who in the Divine nature could do all things as God, was yet unable in His own Person to become God in man. Setting aside then and rejecting your foolish and feeble and dull arguments from earthly things, we ought merely to put credence in straightforward evidence and the naked truth, and to adapt our faith to those witnesses of God alone, whom God sent, and in whose person He Himself, so to speak, preached. For it is right to believe Him in a matter concerning knowledge of Himself, as everything that we know of Him comes from Him Himself, for God could not possibly be known of men, unless He Himself gave us the knowledge of Himself. And so it is right that we should believe everything of Him that we know, from whom comes everything that we know, for if we do not believe Him from whom our knowledge comes, the result will be that we shall know nothing at all, since we refuse to believe Him, through whom our knowledge comes.