Mother of God
When Elizabeth heard Mary’s greeting, the baby leaped in her womb, and Elizabeth was filled with the Holy Spirit. She called out with a loud voice, and said, “Blessed are you among women, and blessed is the fruit of your womb! Why am I so favored, that the mother of my Lord should come to me? For behold, when the voice of your greeting came into my ears, the baby leaped in my womb for joy! Blessed is she who believed, for there will be a fulfillment of the things which have been spoken to her from the Lord!” Mary said, “My soul magnifies the Lord. My spirit has rejoiced in God my Savior, for he has looked at the humble state of his servant. For behold, from now on, all generations will call me blessed.
Luke 1:41-48In context. Mary visiting Elizabeth: Elizabeth calls her ‘the mother of my Lord’, and Mary’s song begins, rejoicing in God ‘my Savior’, who has looked on his servant’s low state.
One more passage: Romans 9:4-5
who are Israelites; whose is the adoption, the glory, the covenants, the giving of the law, the service, and the promises; of whom are the fathers, and from whom is Christ as concerning the flesh, who is over all, God, blessed forever. Amen.
Romans 9:4-5In context. Paul, grieving that most of Israel has not believed, lists what God gave them, ending with Christ, who came from them “as concerning the flesh” and is “over all, God”.
Scripture states it. Elizabeth, filled with the Holy Spirit, calls Mary “the mother of my Lord”, and two verses later uses “the Lord” for God; Paul says Christ is “over all, God”. So Scripture states that the one Mary bore is God the Son. “Mother of God” is not its phrase, and means mother of God the Son in his human birth, never of the Father or of God’s being (what the title does and does not mean).
The Catholic reading
The Council of Ephesus (431) called her Theotokos, God-bearer, to protect the truth that the child she bore is one person, truly God.
“If anyone does not confess that Emmanuel is God in truth, and therefore that the holy virgin is the mother of God”
Council of Ephesus, the first anathema, 431
The Protestant reading
Lutherans confess the title as a statement about Christ. Calvin accepted what it guards but refused the title as feeding superstition, and many Protestants confess the doctrine and seldom use the words.
“Therefore she is truly the mother of God, and nevertheless remained a virgin.”
Formula of Concord, Solid Declaration VIII.24, 1577