Why is Mary called the Mother of God?

Mary is called the Mother of God because she gave birth to Jesus Christ, and Jesus is truly God. The title protects the truth about Christ rather than claiming that Mary created God.

What the title means. Mothers give birth to persons, not merely to bodies or natures. Mary conceived and bore the divine person of the Son, who took a real human nature from her. The child born of Mary is one person, Jesus Christ, with two natures, divine and human. Because that person is God the Son, Mary is rightly called Mother of God, or Theotokos, meaning "God-bearer" (CCC 495).

What it does not mean. Mary is not the source of the Trinity, and she did not exist before God. The Son is eternally begotten of the Father and has always possessed His divine nature. In time, He assumed a human nature and was born of Mary (Jn 1:1, 14; Gal 4:4). Mary is His mother according to His humanity, but the one whom she bore is the eternal Son Himself.

Where Scripture teaches it. When Mary visited Elizabeth, Elizabeth was filled with the Holy Spirit and asked, "How does this happen to me, that the mother of my Lord should come to me?" (Lk 1:43). In Luke’s Gospel, "Lord" is a divine title. Gabriel likewise announced that Mary’s child would be called holy, "the Son of God" (Lk 1:35). Scripture never divides Jesus into a human person born from Mary and a separate divine person.

Why the Church defined it. In 431, the Council of Ephesus defended the title Mother of God against Nestorius, whose teaching threatened to divide Christ. The council affirmed that the eternal Son truly became man and that the divine and human natures are united in His one person. The Council of Chalcedon later confessed the same Lord Jesus Christ as truly God and truly man (CCC 466-467). Denying Mary this title would suggest that the one born from her was someone other than God the Son.

Calling Mary Mother of God is therefore a confession about Jesus. When Catholics use the title, they proclaim that the infant in Bethlehem, the man who died on Calvary, and the risen Lord are one and the same divine person. A simple prayer such as "Holy Mary, Mother of God, pray for us" should lead the heart to adore her Son.