Theodor emphasizes, against Apollinarius, the perfect nature of man in unity with the perfect nature of God. He says: “A complete man, in his nature, is Christ, consisting of a rational soul and human flesh; complete is the human person; complete also the person of the Divinity in him. It is wrong to call one of them impersonal.” This was what finally prevailed in many sections of the East, in everything Monophysite, that only one nature is personal, namely the Divine, and the human is not. Therefore he says: “One should not say that the Logos became flesh.” You remember I came to this again and again already in the Apostolic Fathers. He says this is a vague metaphoric kind of talk and should not be used as a precise formula, but one should say: He took on humanity. “The Logos had not been transformed into flesh.” This transformation, or transmutation, idea was felt by him as pagan, and so he rejected it. But the pagan spirit of superstition wanted to have a transformed God walking on earth. But of course this brought Theodor into a very hard problem. If each side in Christ, the human, and the Divine, are themselves persons, is He not a being with two personal centers? Is He not a combination of two sons, a monster with two heads, as his enemies told him? Theodor tried to show the unity of the two persons. He rejected the unity in essence or nature. In essence they are absolutely different because the Divine nature cannot be confined to an individual man. The Logos, as follows from the Fourth Gospel, is always universally present. Even when Jesus lived, the f lowers were blooming, the animals living, men were walking, culture was going on. All this is Logos. How can the Logos be only the man Jesus?,;, he says;that is impossible. He speaks, therefore, of a unity by the Holy Spirit, which is a unity of grace and will. In this way he establishes in Jesus the analogy to the prophets, who were driven by the Spirit. But it is a unique event because in the prophets the Spirit is limited; in Jesus the Spirit is unlimited.
The union of the two natures started in the womb of Mary. In it the Logos has connected a perfect man with Himself in a mysterious way. This Logos directs the development of Jesus, His inner growth. But it does not do so by coercion. Jesus, as every man, has grace, even unlimited grace. But grace never works through coercion, but through the personal center. In this way Jesus increased in perfection, by the grace of God. So he says we have one person, but the natures are not mixed.
Christianity, History of Christian Thought, Studies, Tillich