The word Lord in Greek (kurios) does not always mean God (theos).
That said, this is a pretty easy one to come to for most folks. It’s important to state what “Mother of God”
does not mean: It does not mean that Mary gave birth to the Trinity, nor that she is older than God the Son, the second Person of the Blessed Trinity. What it
does mean is the following:
Major premise: Jesus was God.
Minor premise: Mary was the mother of Jesus.
Conclusion: Mary was the mother of God.
In order to escape this logical conclusion, you must refute either the major premise or the minor premise. It would seem that most objectors would grant both, however some assert that Jesus was not just one person - He was kinda’ like a schitzophrenic, with a God-within-him and a Him-without-God. This is the heresy known as Nestorianism. (You can learn more about the early heresies
here.)
I believe one of the best “proof texts” would be John 20:28. In John 20:28 the apostle Thomas, upon seeing the resurrected Jesus, said to him, “My Lord and my God!” The Greek text literally reads, “The Lord of me and the God of me” ("
ho kurios mou kai ho theos mou"). Another use where ‘kurios’ means God is in 2 Corinthians 3:16-18, where Paul identifies the Spirit with
ho kurios (“the Lord”): “Whenever a person turns to the Lord the veil is removed. Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. All of us, gazing with unveiled face on the glory of the Lord, are being transformed into the same image from glory to glory, as from the Lord who is the Spirit.”
So it can be demonstrated that Lord and God can be used interchangably, but Lord does not
necessarily mean God (although God is always Lord). It’s like saying “all apples are fruit but not all fruit are apples”. The NT writers typically use Lord (and not God) in reference to Christ in order to keep the persons of the Trinity separate and distinct, and prevent the Modalist heresy (linked above).
Please let me know if further clarification is needed.
God Bless,
RyanL
P.S.
I’m sure you’ve already found it, but there’s also Luke 1:43, where Elizabeth says to Mary: