Q. Is it wrong to refer to Mary as "the mother of God"?
A. Orthodox, Catholics, Anglicans, and even some Protestants have long referred to Mary as the Theotokos, which literally means "God-bearer". In contemporary usage this Greek word is often imprecisely translated "mother of God'.
Many Christians bristle at this phrase. In the early centuries of the Church a man named Nestorius adamantly rejected referring to Mary as the theotokos. While he firmly believed that Christ has two natures (one divine, one human), he maintained that only the 'human' side of Christ was capable of being born. So while Mary was truly the mother of Jesus, and even the mother of the Messiah, she cannot not be called the 'bearer of God'. He maintained that divine nature, by definition, cannot have a mother. We often hear such language in contemporary preaching. I remember hearing one Evangelist chiding Catholicism on this point, stating in a dramatic fashion that it is a horrible heresy to call Mary the mother of God.
I can sympathize with Nestorius (and this Evangelist), but I must disagree with him. Certainly Mary is not the mother of God in an eternal sense. But Scripture clearly maintains that Mary gave birth to God's divine son. Consider the following verses:
John 1:14, "The Word became flesh and made his dwelling among us."
Jesus' person cannot be split into two 'sections'. It is not as if He is 1/2 God and 1/2 man. To say that Mary gave birth only to the human portion of Jesus is to misunderstand the incarnation. In fact, in 451 (the Council of Ephesus), the historic Church considered such belief heresy (so, our Evangelist friend above was the heretic, not the Catholics--at least on this point). As an illustration, your genetic make-up comes 50/50 from your mom and dad. You cannot divide out separately what is from your Dad. Its all mixed together. That is an imperfect illustration but it demonstrates something similar about Jesus. Jesus truly has two natures (God/man), but He is one person that cannot be divided.
Galatians 4:4-5, "But when the set time had fully come, God sent his Son,
born of a woman, born under the law, to redeem those under the law,
that we might receive adoption to sonship."
The apostle Paul clearly maintains that Jesus, the divine son of God, was born of a woman. If this is unthinkable to you I would suggest that is the entire point. The incarnation is radical. It brings about that thing which is not supposed to be possible, yet with God, all things are possible (Matt 19:26). Cyril of Alexandria once wrote, "I am amazed that there are some who are entirely in doubt as to whether Mary should be called God-bearer or not. For if our Lord Jesus Christ is God, how is Mary who gave him birth not a God-bearer?"
Does this mean that we should agree with the Catholic view of Mary? On this issue, most certainly. The miracle of the incarnation isn't just that a virgin conceived (though that is obviously miraculous). The greatest miracle of the incarnation is that God became man while remaining God. Col 2:9 is adamant: "For in Christ all the fullness of the Deity lives in bodily form". Mary gave birth not just to a human Jesus, but to a fully human and fully divine Jesus.
Yet we strongly disagree with other ways the Catholics/Orthodox view Mary. While Scripture tells us she is blessed (Luke 1:42), she was still a sinner who needed a Savior. She is not holy because she gave birth to Jesus. She is holy because Jesus saved her from her sins.
In summary. It is heresy to deny that Mary is the mother of God. But is also heresy to pray to her or venerate/worship her.