And since the holy Virgin brought forth corporally God made one with flesh according to nature, for this reason we also call her Mother of God, not as if the nature of the Word had the beginning of its existence from the flesh. (St. Cyril, third synodical letter to Nestorius)
but the flesh was not who he was
This is a denial of the incarnation and of the whole salvific economy. If the Word did not become flesh, who suffered and died and rose again for the sake of our salvation?
"Which none of the princes of this world knew; for if they had known it, they would never have crucified the Lord of glory." (1 Cor. 2:8) But according to you, the Lord of majesty - the Son of God, was not crucified, just as he was not born of his mother the Virgin. Apparently, for you, all that was crucified was the "human form" of God the Son (rather, the human form united to the Son in name only), not God the Son himself in his human nature.
But when our Lord and Saviour himself was by his questions instructing the faith of the disciples, he said, “Whom do men say that I the Son of Man am?” And when they had mentioned various opinions held by others, he said, “But whom say ye that I am?” that is, “I who am Son of Man, and whom you see in the form of a servant, and in reality of flesh, whom say ye that I am?” Whereupon the blessed Peter, as inspired by God, and about to benefit all nations by his confession, said, “Thou art the Christ, the Son of the living God.” Not undeservedly, therefore, was he pronounced blessed by the Lord, and derived from the original Rock that solidity which belonged both to his virtue and to his name, who through revelation from the Father confessed the selfsame to be both the Son of God and the Christ; because one of these truths, accepted without the other, would not profit unto salvation, and it was equally dangerous to believe the Lord Jesus Christ to be merely God and not man, or merely man and not God. But after the resurrection of the Lord—which was in truth the resurrection of a real body, for no other person was raised again than he who had been crucified and had died—what else was accomplished during that interval of forty days than to make our faith entire and clear of all darkness? For while he conversed with his disciples, and dwelt with them, and ate with them, and allowed himself to be handled with careful and inquisitive touch by those who were under the influence of doubt, for this end he came in to the disciples when the doors were shut, and by his breath gave them the Holy Ghost, and opened the secrets of Holy Scripture after bestowing on them the light of intelligence, and again in his selfsame person showed to them the wound in the side, the prints of the nails, and all the flesh tokens of the Passion, saying, “Behold my hands and my feet, that it is I myself; handle me and see, for a spirit hath not flesh and bones, as ye see me have:” that the properties of the Divine and the human nature might be acknowledged to remain in him without causing a division, and that we might in such sort know that the Word is not what the flesh is, as to confess that the one Son of God is both Word and flesh. (St. Leo the Great, Tome to the Council of Chalcedon)