Mother of God
It is good to grasp that the PERSON to whom MARY gave birth was the CHRIST [Luke 2:7] who is truly GOD [1 Tim. 3:16, Heb. 1:4-8, Jn. 13:19, Jn. 1:1-14, etc] & truly HUMAN [1 Tim. 2:6, Acts 2:22]. If Christ is God then He must have existed before Mary- no contradiction.
When He became man His being God was not lost, He only partook of Mary’s human flesh. So, He was conceived & given birth to by Mary & He is both HUMAN & DIVINE.
So the question who is Christ the answer is God/LORD. Who is Mary? Answer is, the mother of Christ & who again Christ is? Answer= God. Mary is therefore the Mother of God not because she existed before God but because SHE carried in HER womb the very PERSON of GOD for 9 months & most especially SHE GAVE BIRTH to this PERSON.
So the title Mother of God is actually an ECHOING of the TRUTH of Jesus’s DIVINITY.
Furthermore, let the Bible speak on this matter more: St. Elizabeth to Mary= “But why am I so favored, that the mother of my Lord should come to me?” [Luke 1:43]. Just read in this gospel what the word “LORD” means:P but for further clarification to its meaning let us read Acts 10:36: “You know the message God sent to the people of Israel, telling the good news of peace through Jesus Christ, who is LORD OF ALL.”
The word Theotokos is Greek which means "God-bearer". The Council of Ephesus in
A.D. 431 DECLARED Mary the Mother of God. This was to safeguard the divinity of Christ, which was being attacked by the Nestorians heretical group. Mary is the Mother of God the Son since Jesus Christ was God in the flesh.