For we do not divide up the words of our savior in the gospels among two hypostases or persons. For the one and only Christ is not dual, even though he be considered to be from two distinct realities, brought together into an unbreakable union. In the same sort of way a human being, though he be composed of soul and body, is considered to be not dual, but rather one out of two. Therefore, in thinking rightly, we refer both the human and divine expressions to the same person. For when he speaks about himself in a divine manner as "he that sees me sees the Father," and "I and the Father are one," we think of his divine and unspeakable nature, according to which he is one with his own Father through identity of nature and is the "image and impress and brightness of his glory." But when, not dishonouring the measure of his humanity, he says to the Jews: "But now you seek to kill me, a man who has spoken the truth to you," again no less than before, we recognise that he who, because of his equality and likeness to God the Father is God the Word, is also within the limits of his humanity. For if it is necessary to believe that being God by nature he became flesh, that is man ensouled with a rational soul, whatever reason should anyone have for being ashamed at the expression uttered by him should they happen to be suitable to him as man? For if he should reject words suitable to him as man, who was it that forced him to become a man like us? Why should he who submitted himself to voluntary self-emptying for our sake, reject expressions that are suitable for such self-emptying? All the expressions, therefore, that occur in the gospels are to be referred to one person, the one-enfleshed hypostasis of the Word. For there is one Lord Jesus Christ, according to the Scriptures....
|Mary, Mother of God, with her Son|
1. If anyone does not confess Emmanuel is God in truth, and therefore that the holy virgin is the mother of God (for she bore in a fleshly way the Word of God become flesh, let him be anathema.
2. If anyone does not confess that the Word from God the Father has been united by hypostasis with the flesh and is one Christ with his own flesh, and is therefore God and man together, let him be anathema.
[Third letter of Cyril to Nestorius, read at the council of Ephesus and included in the proceedings. Two of the twelve anathemas proposed by St. Cyril and accepted by the Council of Ephesus are included here.]
This is the whole point of the Church calling the Blessed Virgin Mary the "Mother of God." Our Lord Jesus was one person with a divine nature and a human nature. He was not two separate persons residing in one body. Just as man and his soul are one and inseparable, so God and man in the person of Jesus Christ Our Lord is inseparable. He is God. If you believe that Jesus was and is God, then why would there be an objection to calling Mary, His mother, the Mother of God?
[Added 8 Sep 2010, 9:45AM: Here is an online source of a translation of the documents of the Council of Ephesus, 431 AD: http://www.ewtn.com/library/COUNCILS/EPHESUS.HTM It looks like the same translation as the hard copy I quoted from here.]