As posted on gotquestions.org
My cmnt: As we once again enter Advent and the Christmas season I thought this would be a good time to address this question.
The phrase mother of God traces back to the third century and continues to be used in some liturgical churches, including the Roman Catholic Church. One of the topics at the Council of Ephesus in AD 431 was the use of the Greek term Theotókos (literally, “God-bearer” or “the one who gives birth to God”) in reference to Mary. That council’s use of Theotokos was meant to counter the heresy of Nestorianism, which cleaved the nature and person of Christ in two: Christ’s human and divine natures were completely divergent and unconnected. According to Nestorius, Mary gave birth to Christ but not to God; Mary was the mother of His humanity, which was totally distinct from His divinity—Jesus was two persons sharing one body, essentially. The Council of Ephesus affirmed the full deity of Christ and unity of His person by saying that Mary did indeed bear God in her womb. Mary is the “mother of God” in the sense that, since Jesus is God and Mary is the mother of Jesus, she is the mother of God. The Word became flesh (John 1:14), and Mary mothered Him.
We should distinguish the term Theotokos from mother of God, because there is a subtle yet important difference. The term mother of God could be taken wrongly as implying that Mary was the source or originator of God, similar to how Juno was the mother of Vulcan in Roman mythology. Of course, Christianity teaches that God is eternal and that Jesus Christ has a pre-existent, divine nature. The idea that Mary is the mother of God in the sense that she was the source of God or somehow predated God or is herself part of the Godhead is patently unbiblical.
The term Theotokos, on the other hand, is more specific and less open to being misconstrued. Theotokos simply implies that Mary carried God in her womb and gave birth to Him. Mary was the human agent through whom the eternal Son of God took on a human body and a human nature and entered the world. The term Theotokos was a succinct expression of the biblical teaching of the Incarnation, and that is how the Council of Ephesus used the word. Mary is the “God-bearer” in that within her body the divine person of God the Son took on human nature in addition to His pre-existing divine nature. Since Jesus is fully God and fully man, it is correct to say that Mary “bore” God.
Even though the term Theotokos was originally used to help explain the Incarnation, many people today use the term, or the related mother of God, to communicate something different. Through the years, many legends accumulated around the person of Mary, and she became an object of worship in her own right. About 350 years after the Council of Ephesus used the term Theotokos in reference to Mary, the Second Council of Nicaea declared, “We honor and salute and reverently venerate . . . the image of . . . our spotless Lady the all-holy mother of God.” This shows the trend within the Roman Church to move from a focus on the Incarnation of God to a veneration of the “Mother of God,” even to the point of honoring her images and praying to her as the “Queen of Heaven,” “Benefactress,” and “Mediatrix.” The necessity of such veneration is not implied by the term Theotokos, but some people wrongly infer it.
Roman Catholic leaders teach their followers to go to Mary to find help in their time of need: “From the most ancient times the Blessed Virgin has been honored with the title of ‘Mother of God,’ to whose protection the faithful fly in all their dangers and needs” (Catechism of the Catholic Church, Part 1, Section 2, Chapter 3, Article 9, Paragraph 6, 971). The Eastern churches still use the term Theotokos, and they sing hymns called theotokia to Mary. This portion of a theotokion is from the liturgy of the Coptic Orthodox Church: “You are the pride of virgins, O Mary the Theotokos. / You are the soul’s city, where the Most High lived, who sits upon the throne, of the Cherubim. . . . / O Virgin Mary, the holy Mother of God, the trusted advocate, of the human race. / Intercede on our behalf, before Christ whom you have born, that He may grant unto us, the forgiveness of our sins” (from The Friday Theotokia – Watos).
These views of Mary represent a theological shift away from Christ as our sole Redeemer and Intercessor (1 Timothy 2:5) and an overemphasis on Mary as the “Mother of God.”