"Let us understand well and clearly the mystery of the Incarnation. If you say that he united himself to man in such as way that his Divinity took the place and fulfilled the duties of the soul in the body, you have only a shell inhabited by the Lord and you have not perfect man, but only a body without a soul. That would be an error of many modern writers.
If you say that the spirit of God dwelt within the body born of Mary, you would have a great prophet of God, and a creature not different from the prophets of old; but he would not be God, he would only come in the spirit of God, and that was the error of the Cerinthians of the first century [and the Arians in the fourth century, if I understand properly – Funky].
If you say that he had no body born of Mary, but that his body was formed of thin atmosphere like a vision, you fall into the erros of the Phantasiasts and Docetists of the early ages.
If you say that he was only a man born of Mary and of Joseph, you fall into the errors of the Ebionites and the Protenites.
If you say that his two natures, the nature of God and the nature of man, were combined in him so as to make two persons, different from one another, you are wrong, you are following the false teachings of the Nestorians.
If you say that his whole human will was absorbed into the Godhead, you fall into the error of the Monothelites.
If you say that he had only one nature, you fall into the errors of the Eutychians.
Now the true doctrine is this: That there was a human nature void without a human person, but, in place of his human person, there was placed the Second Person of the Holy Trinity, and there was God and man united in the Person, and there was in him two natures, the one of God and the other of man, but there was in him only one person, the Person of the Holy Trinity. And, as all the action of a man and all things that he does are referred to him, uniting both the body and soul, so all the works on God and everything that he did were referred to his Person. And the one who is responsible for all these things is is the Second Person of the Holy Trinity; and, therefore, the acts of his soul, and the actions of his body, and the actions of his mind, and his sufferings, and his privations, and all the things that he did, are the actions, and the works, and the operations, not of a human being, but the Second Person of the Trinity. Therefore, these are the actions of God." (Ch. 7, pp. 108-109)