Tim Staples explains how the title Mother of God has it’s foundation in Scripture. It all goes back to the story of the Visitation when Elizabeth calls Mary “Mother of my Lord.”

First, when Elizabeth “exclaimed with a loud cry… why is this granted me, that the mother of my Lord should come to me” (Luke 1:42-43), Mary was revealed to be the New Testament Ark of the Lord.Elizabeth’s words make this clear as they hearken back to a text from II Samuel 6:9 wherein David exclaims concerning the Old Covenant “ark of the Lord:”

And David was afraid of the LORD that day; and he said, “How can the ark of the LORD come to me?”

If this one parallel leaves you unconvinced, there are more that may tip the scale for you. St. John the Baptist “leaped for joy” at the salutation of Mary (Luke 1:44), just as King David “danced before the Lord” in the ark of the Lord in II Samuel 6:14. Moreover, Mary “remained with [Elizabeth] for three months (Luke 1:56),” just as “the ark of the Lord remained in the house of Obededom the Gittite for three months” in II Sam. 6:11.

Read the whole article here. Go here for more on Mary, Mother of God. Here is a longer apologetics article Mother of God-Defender of Orthodoxy.