We've been learning (here and here) about who Christ is, in particular, in light of the Chalcedonian use of theotokos, "God-bearer," about Mary. Now it is time to go back to the sources.
Why did Nestorius oppose the term theotokos? He explains in a letter to Celestine of Rome.
We have found no slight corruption of orthodoxy among some of those here, which we have treated with both sternness and gentleness. It is no small error, but similar to the corruption of Apollinaris and Arius,blending together the Lord's appearance as man int o a kind of confused combination--so much so that certain of our clergy...err like heretics, and openly blaspheme God the Word consubstantial with the Father, as if he took his beginning from the Christ-bearing Virgin, and grew up with his temple and was buried with [it]in the flesh; they even say that his flesh after the resurrection did not remain flesh, but was changed into the nature of Godhead. To speak briefly, they refer to the Godhead of the Only-begotten to the same origin as the flesh joined [with it], and kill it with the flesh, and blasphemously say that the flesh joined with the Godhead was turned into deity by the deifying Word, which is nothing more nor less than to corrupt both. They even dare to treat of the Christ-bearing Virgin in a way as along with God, for they do not scruple to call her theotokos, when the holy and beyond-all-praise Fathers at Nicaea said no more of the holy Virgin than that our Lord Jesus Christ was incarnate of the Holy Spirit and the Virgin Mary--not to mention the Scriptures, which everywhere, both by angels and apostles, speak of the Virgin as mother of Christ, not of God the Word....
Many of those who had gone astray have by the grace of the Lord repented, learning from us that what is born is properly consubstantial with the parent, and that it was to the creature of the Lord's humanity, joined with God, of the Virgin by the Spirit, that what was seen among men was committed. If anyone wants to use this word theotokos with reference to the humanity which was born, joined to God the Word, and not with reference to the parent, we say that this word is not appropriate for her who gave birth, since a true mother should be of the same essence as what is born of her. But the term could be accepted in consideration of this, that the word is used of the Virgin only because of the inseparable temple of God the Word which was of her, not because she is the mother of God the Word--for none gives birth to one older than herself. (Translated by Edward R. Hardy in Christology of the Later Fathers)
You can plainly see that Nestorius' concern was to uphold and defend the orthodox doctrine of Christ. He fought against a real error, namely, deifying the flesh of Jesus. But in doing so he misconstrued the union of Christ's divine and human natures. According to Nestorius' reasoning, it was only Christ's humanity which was born of Mary, although this humanity was indeed "joined with" Christ's deity. By doing so, he created a fissure in Christ's person. In my next post on this topic, I'll put up Cyril of Alexandria's response to Nestorius.