The belief that Mary is the mother of God is not unique to Catholicism. The vast majority of Christians (Eastern Orthodox, Oriental Orthodox, Lutherans, and Anglicans) accept this dogma, with only a minority in the Protestant community objecting.
One text appealed to in support of this belief is Luke 1:43. There, Elizabeth, inspired by the Holy Spirit, exclaims to Mary, who just arrived in her presence, “Why is this granted me, that the mother of my Lord should come to me?” Since Elizabeth was a good Jew, and Jews normally used the word Lord in the place of the tetragrammaton (God’s name), YHWH, Elizabeth is calling Mary the mother of God. Therefore, we have a possible biblical foundation for the dogma of Mary, mother of God (see also CCC 448, 495, 2677).
There are many comebacks that Protestants have to the belief in Mary as the mother of God. But there’s really only one counterargument to Luke 1:43. It targets the assumption that “Lord” is intended by Elizabeth to refer to Almighty God.
Protestant Bible scholar Walter L. Liefeld argues that we shouldn’t interpret this as a reference to Mary, “mother of God.” His alternative interpretation is that Elizabeth was referring to Jesus as the Messiah:
Nowhere in the [New Testament] is Mary called “mother of God.” Deity is not confined to the person of Jesus (we may say, “Jesus is God,” but not all of “God is Jesus”). She was, however, the mother of Jesus the Messiah and Lord (834).
The evidence he gives is the fact that Luke frequently uses “Lord” as a title—95 out of 166 occurrences in the synoptics—and not every one is charged with a divine meaning. Moreover, Liefeld argues, Jesus is called “Lord” elsewhere in the Lukan birth narrative in a non-divine way (“For to you is born this day in the city of David a savior, who is Christ the Lord”—Luke 2:11).
It’s not clear whether Luke uses “Lord,” in reference to Jesus, in 2:11 in a divine or non-divine way. There is nothing in the text that suggests either interpretation. Liefeld simply asserts its divine use without argumentation. Given such ambiguity, we can dismiss this text as evidence for Liefeld’s conclusion.
There’s no doubt, however, that the Greek word translated “Lord,” kurios, is used in a non-divine way in the New Testament (e.g., 1 Cor. 8:5), and even by Luke (e.g., 12:36, 37, 42, 43, 45, 46, 47). However, it’s not the word by itself that indicates that Mary is the mother of God. It’s how Luke sees Elizabeth using it.
There are several details that indicate that Luke is drawing a parallel between Mary and the Old Testament Ark of the Covenant. Take Elizabeth’s words themselves, for example. They almost perfectly mirror David’s in 2 Samuel 6:9, when he says in the presence of the ark, “How can the ark of the Lord come to me?” Other parallels include John the Baptist leaping for joy in the presence of Mary in Luke 1:44 and David “making merry” before the ark in 2 Samuel 6:5. According to Luke 1:39, Mary remains with Elizabeth for three months, similar to how the ark remained in the house of Obededom for the same amount of time according to 2 Samuel 6:11.
Since Luke is paralleling Elizabeth’s “mother of my Lord” with David’s “ark of the Lord,” it stands to reason that Luke intends for us to take Elizabeth’s cry as a reference to almighty God. “Lord” in the phrase “ark of the Lord” wasn’t a reference to the Messiah. The ark was the ark of almighty God. Therefore, we have good reason to interpret Luke 1:43 as a reference to Mary being the mother of God, contrary to Liefeld’s claim.
Now, a Protestant at this juncture might argue that if we take some parallels with the ark, then we need to take all of them. Protestant apologist James White poses this argument:
Must Mary have been stolen by God’s enemies for a time, so that she could be brought back to the people of God with great rejoicing (2 Sam. 6:14-15)? Who was Mary’s Uzzah (2 Sam. 6:3-8)? [Catholic apologist Patrick] Madrid draws a further parallel between the three months the ark was with Obededom and the three months Mary was with Elizabeth. What, then, is the parallel with David’s action of sacrificing a bull and a fattened calf when those who were carrying the ark had taken six steps (2 Sam. 6:13) (205)?
White charges that the use of Mary as the new Ark of the Covenant is violating rules of scriptural interpretation, since he perceives it as picking and choosing “those aspects of Mary’s life [a Catholic] wishes to parallel in the ark and those which he does not.”
In response, there’s a hidden principle on which White seems to base his argument—namely, that some parallels requires all parallels. But this is simply false. That’s not how prophetic foreshadowing or intertextuality works. The New Testament authors themselves, for example, don’t honor the principle contained in White’s argument. Consider the first two verses of Hosea 11:
When Israel was a child, I loved him, and out of Egypt I called my son. The more I called them, the more they went from me; they kept sacrificing to the Baals, and burning incense to idols.
Matthew takes the phrase “out of Egypt I called my son” in the first statement as a prefigurement of the baby Jesus’ return from the flight to Egypt (Matt. 2:15). Yet Matthew does not intend the latter part of the passage to refer to Jesus: Jesus did not go away from God, sacrifice to the Baals, and burn incense to their images.
There are numerous examples of this in the New Testament’s use of the Old. Whenever prophetic foreshadowing is in play, some elements foreshadow, and some don’t. There are continuities and discontinuities. If the New Testament authors employ this type of hermeneutic when relating the Old Testament to the New, it’s legitimate for Catholics to do the same.
So neither of the above counterarguments undermines a Christian’s appeal to Luke 1:43 as biblical evidence for Mary as mother of God. This being the case, we have confirmation by the Holy Spirit himself, who inspired Elizabeth to utter the words “mother of my Lord,” that Mary is indeed the Theotokos, the God-bearer.