I begin these reflections on the Marian topic with which I am most comfortable: calling Mary the “Mother of God” or (in the language of the early Church) the theotokos (God-bearer). There are several reasons for my affirmation of this statement.
First, calling Mary the “Mother of God” is Christological before all else. That is, the essential nature of the theology being conveyed by this title for Mary involves the divinity of her Son. In fact, calling Mary the bearer of God says far more about who her Son is (Jesus, not just any old Messiah, but the Son of the Living God who is fully divine) than it does about Mary. As Cyril of Alexandria explains in his Third Letter to Nestorius:
“And since the holy Virgin hath borne after the Flesh God united personally to the Flesh, therefore we do say that she is also Mother of God, not as though the Nature of the Word had the beginning of Its existence from flesh, for It was in the beginning and the Word was God, and the Word was with God [John 1:1], and is Himself the Maker of the ages, Co-eternal with the Father and Creator of all things.”
Of course, being the person who carried the incarnate Son around for nine months is worthy of at least some measure of honor. Yet the fact remains that the title “Mother of God” was given to Mary in order to protect the truth about who Jesus of Nazareth truly was (and is)—the Divine Son of Yahweh.
Second, calling Mary the “Mother of God” is –in an important sense—Biblical. Several months back I heard a sermon which argued against calling Mary the Mother of God because “nowhere in the New Testament do we ever see that phrase [Mother of God]. She is never called the Mother of God.” If we adhere only to the letter of our English translations of the New Testament (we could call this a “wooden literal” interpretation of the text), this statement is true. Nowhere is Mary called the Mother of God.
But to make this type of reading of the Bible the basis of Christian theology is deeply problematic. There are plenty of other phrases that never occur in the New Testament. “Trinity” for one, “fully God and fully man” for another, and “the inspired and inerrant Bible is the final rule for life and faith” as a final example. The New Testament does make two clear claims, however: 1) Jesus is God and 2) Mary is the Mother of Jesus. According to the internal grammar and meaning of the Bible, then, Mary is the “Mother of God.” Fortunately, the message (if not the letter) of the New Testament allows us to understand who Jesus is—the divine Son of God—and, subsequently, who His mother was as well—the Mother of God incarnate.
The third reason that I confess Mary to be the “Mother of God” is Historical. In the fifth century, a bishop named Nestorius (or at least some of his followers) suggested that Mary be called the Christotokos (Christ-bearer), because the person that she was bearing—while the Christ—was not actually God. For the heresy that came to be called Nestorianism, Jesus Christ and the Divine Logos were two separate persons, joined during portions of Jesus’ life, but ultimately distinct. This view—that Mary did not bear the Divine Logos—was condemned at the Council of Ephesus in 431 CE. Thus, I believe that Mary should be called the “Mother of God” because that is part of the faith that has been handed down to me by earlier followers of Jesus.
While this term may feel unfamiliar to some (contemporary, American) Christians and may be in need of explanation in some quarters, Mary should be understood as the “Mother of God” (i.e., the Mother of the Divine Logos incarnate in Jesus the Christ) by Christians. Accordingly, it is entirely fitting that she be called the theotokos. To conclude, I again cite Cyril of Alexandria, this time from his Letter to the Monks of Egypt:
“I have been amazed that some are utterly in doubt as to whether or not the Holy Virgin is able to be called the Mother of God. For if our Lord Jesus Christ is God, how should the Holy Virgin who bore him not be the Mother of God?”
 There is some level of debate concerning whether or not Nestorius was actually Nestorian; I tend to think that he was actually not a heretic, even though the belief bearing his name is.