The Blessed Virgin Mary
Mary, the Mother of God
“Mother of God” (Theotokos, ‘God-bearer’) doesn't mean Mary is the source of the divine nature. It means the one she bore is a divine Person — Jesus is God, and Mary is his mother. Deny the title and you end up denying that Jesus is God.
Scripture already gives her the title
Under the inspiration of the Holy Spirit, Elizabeth calls Mary “the mother of my Lord” — ‘Lord’ (Kyrios) being the divine name. The child in her womb is God the Son.
The one she bore is God made flesh
The Word who ‘was God’ became flesh; God sent his Son ‘born of a woman.’ The person born of Mary is the eternal God. So she is truly the mother of God the Son according to his humanity.
The title defends Jesus, not just Mary
The Council of Ephesus (AD 431) defined Theotokos precisely to protect the truth that Jesus is one divine Person — not a God-part and a man-part loosely joined. It is first of all a statement about Christ.
Common objections
“Mary is only the mother of Jesus' human nature.”
We never say a mother is 'mother of a nature' — your mother is the mother of YOU, a person, even though she didn't create your soul. Jesus is a single divine Person; whoever is his mother is the mother of that Person, who is God.
“Calling her ‘Mother of God’ makes her older than God.”
It says nothing about origins. As God, the Son is eternal and un-originated; as man he was born in time of Mary. Motherhood concerns the person born, not the beginning of his divinity.
Scripture quoted verbatim from the World English Bible (public domain).