4 Whence did it occur to you, sirs, to say that the Body is of one Essence with the Godhead of the Word? For it is well to begin at this point, in order that by showing this opinion to be unsound, all the others too may be proved to be the same. Now from the divine Scriptures we discover nothing of the kind. For they say that God came in a human body. But the fathers who also assembled at Nicæa say that, not the body, but the Son Himself is coessential with the Father, and that while He is of the Essence of the Father, the body, as they admitted according to the Scriptures, is of Mary.
Either then deny the Synod of Nicæa, and as heretics bring in your doctrine from the side; or, if you wish to be children of the fathers, do not hold the contrary of what they wrote. For here again you may see how monstrous it is: If the Word is coessential with the body which is of earthly nature, while the Word is, by your own confession, coessential with the Father, it will follow that even the Father Himself is coessential with the body produced from the earth. And why any longer blame the Arians for calling the Son a creature, when you go off to another form of impiety, saying that the Word was changed into flesh and bones and hair and muscles and all the body, and was altered from its own nature?
For it is time for you to say openly that He was born of earth; for from earth is the nature of the bones and of all the body. What then is this great folly of yours, that you fight even with one another? For in saying that the Word is coessential with the Body, you distinguish the one from the other, while in saying that He has been changed into flesh, you imagine a change of the Word Himself. And who will tolerate you any longer if you so much as utter these opinions? For you have gone further in impiety than any heresy.
For if the Word is coessential with the Body, the commemoration and the work of Mary are superfluous, inasmuch as the body could have existed before Mary, just as the Word also is eternal: if, that is, it is as you say co-essential with the Body. Or what need was there even of the Word coming among us, to put on what was coessential with Himself, or to change His own nature and become a body? For the Deity does not take hold of itself, so as to put on what is of its own Essence, any more than the Word sinned, in that it ransoms the sins of others, in order that changing into a body it should offer itself a sacrifice for itself, and ransom itself.
Source: Letters (New Advent)