Calling St. Mary “the mother of God” is not only a matter of creed related to St. Mary, but also to our Lord Jesus Christ.
First, calling the Blessed Virgin Mary “the mother of God” essentially means that the child who is conceived in her womb by the power of the Holy Spirit, carried in her body and born into this world, is God. Consequently, the title of “mother of God” highlights the pivotal truth of the Orthodox, and Catholic, Church’s Faith which is that God has, in Jesus Christ, manifested in human nature, chosen to be born into this world and lived a real, human life. So, it was through her that Jesus, who is God, was born. It doesn’t mean that she is the creator of God or the world. However, she was full of grace that God chose her to conceive His Son, Jesus Christ, to be born through her.
Second, in the Old Testament, the prophecy of Isaiah speaks of a virgin mother of Immanuel, a name that means “God is with us”. In Isaiah 7:14 “Therefore the Lord Himself will give you a sign: Behold, the virgin shall conceive and bear a Son, and shall call His name Immanuel.” Later in Isaiah 8:8-9 Immanuel is referred to as the future Savior of his people. Here the prophet foretells an extraordinary future sign: that a virgin, without the cooperation of a man, will give birth to a child whose name means “God is with us.” Also, the prophecy of Micah 5:2-3 foretells the birth of the Savior in Bethlehem from a woman who will “bring forth” the “ruler of Israel” and that she will be this ruler’s mother.
Third, the New Testament clearly mentions the greeting of Elizabeth to Saint Mary as “mother of my Lord”. In Luke 1:43, when Saint Mary’s relative, Elizabeth, greeted her shortly after she had conceived The Lord, and said “And why is this granted to me, that the mother of my Lord should come to me?” So, it is stated in the Bible that Saint Mary was called the “mother of the Lord” which has the same meaning of “mother of God” as The Lord Jesus Christ is God.
So, both the Old Testament and the New Testament reveal to us that Saint Mary is the mother of God through prophecies and direct references.