Luke 1:43 (RSV) And why is this granted me, that the mother of my Lord (kurios) should come to me?
John 20:28 Thomas answered him, “My Lord (kurios) and my God (theos)!”
“Lord (kurios) God (theos)”: Lk 1:6, 32, 68; 4:8, 12; 10:27; 20:37.
Therefore, “Lord (kurios) equals God (theos).
Jesus is called both in Jn 20:28.
Mary is mother of the Lord (Lk 1:43).
Therefore, she is the mother of God, since Lord=God.
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We don’t say of mothers that they are the mother of their child’s body, but of the child, and the child has a body and a soul. They didn’t create the soul; God did.
Likewise, with Jesus, Mary was the mother of Jesus, Who is God the Son. Thus, she is the Mother of God. It’s wrong and even illogical to say she was the mother of His body. No; she was the mother of the Divine Person, Jesus, Who had a human nature and also a Divine Nature (that she had nothing to do with). But she is still the mother of the Person, regardless of that, as any mother is the mother of a person who has a soul directly created by God.