Today (Jan. 1st, 2014) is the feast day of Mary, the “Mother of God,” not because she has been around for eternity or that she gave birth to God or anything silly like that, (duh!), but because Mary was the mother of Jesus, who is God (Jn. 1:1).
Jesus was fully human and fully divine, and it is impossible to separate His two natures. Even though there are two natures, He is still only one person, a divine person. Therefore, Mary is not the mother of just His human side, or just of His divine side as some have falsely supposed, nor is she just the mother of a body. She was the Mother of Jesus Christ, whole and entire, the second person of the Trinity, of the Godhead.
That is why we don’t call Mary just the mother of Christ, or Jesus’ mom, of the mother of Jesus, etc. Not that those titles aren’t true, but, they are not enough. They are insufficient. Mary as the mother of God is most accurate and protects Jesus’ divinity. The doctrine of Theotekos (Mother of God) is not about Mary so much as it is about Jesus. It protects the divinity of Christ from people who would like to separate his Hypostatic union like Nestorius did in the 431 A.D.
He stated that Christ was two separate persons, a human person and a divine person, and that Mary was only the mother of his human side. Then, Cyril of Jerusalem stated at the Council of Ephesus that: 1. Mary was the mother of Jesus; 2. Jesus was God; 3. Therefore, Mary was the mother of God.
This is what we celebrate. Jesus was not just a human and divine person. He is God, a divine person who became human for us, to die for us, and to reconcile us back to Himself. He was fully God and fully man, one person.