8 These things being thus demonstrated, it is superfluous to touch upon the other points, or to enter upon any discussion relating to them, since the body in which the Word was is not coessential with the Godhead, but was truly born of Mary, while the Word Himself was not changed into bones and flesh, but came in the flesh. For what John said, 'The Word was made flesh,' has this meaning, as we may see by a similar passage; for it is written in Paul: 'Christ has become a curse for us.'
And just as He has not Himself become a curse, but is said to have done so because He took upon Him the curse on our behalf, so also He has become flesh not by being changed into flesh, but because He assumed on our behalf living flesh, and has become Man. For to say 'the Word became flesh,' is equivalent to saying 'the Word has become man;' according to what is said in Joel: 'I will pour forth of My Spirit upon all flesh;' for the promise did not extend to the irrational animals, but is for men, on whose account the Lord has become Man.
As then this is the sense of the above text, they all will reasonably condemn themselves who have thought that the flesh derived from Mary existed before her, and that the Word, prior to her, had a human soul, and existed in it always even before His coming. And they too will cease who have said that the Flesh was not accessible to death, but belonged to the immortal Nature. For if it did not die, how could Paul deliver to the Corinthians 'that Christ died for our sins, according to the Scriptures,' or how did He rise at all if He did not also die?
Again, they will blush deeply who have even entertained the possibility of a Tetrad instead of a Triad resulting, if it were said that the Body was derived from Mary. For if (they argue) we say the Body is of one Essence with the Word, the Triad remains a Triad; for then the Word imports no foreign element into it; but if we admit that the Body derived from Mary is human, it follows, since the Body is foreign in Essence, and the Word is in it, that the addition of the Body causes a Tetrad instead of a Triad.
Source: Letters (New Advent)