This has been the most attacked by some mainstream Protestants, that Mary has been called, "The Mother of God". For them, God is eternal, with no beginning and no end (cf. Psalm 90:2) and that He is the Alpha and Omega (Isaiah 44:6, cf. Revelation 1:17). But the Church has an explanation to this dogma:
Correction # 1: When we say Mary is the Mother of God, we do not mean that she is the Mother of the Father Almighty. Remember, the Father is eternal, with no beginning or end. He is eternally God forever (Psalm 90:2, Jeremiah 10:10).
Correction # 2: When we say Mary is the Mother of God, we do not mean she is the Mother of the Son in His divinity. Remember, before the world or even Mary existed, the Son was eternally God who was with the Father. St. John calls Him, "The Word" who is Jesus Christ (John 1:1). And Hebrews testify that He, in the beginning created the heavens and the earth (Hebrews 1:10).
Correction # 3: When we say Mary is the Mother of God, we do not mean that she is the Mother of the Holy Spirit. Remember, the Holy Spirit came from the Father and the Son (Luke 24:49, John 14:16) and not from Mary.
Correction # 4: When we say, Mary as the Mother of God, we DO NOT MEAN she is the Mother of the Blessed Trinity. Remember, the Blessed Trinity was there from the very beginning before Mary was. Scripture itself attests this fact:
Genesis 1:1-2, "In the beginning, God created the heavens and the earth. And the earth was without form and void, and darkness was upon the face of the deep, and the Spirit of God hovered upon the face of the waters"
John 1:1, "In the beginning was the Word, and the Word was with God, and the Word was God"
Mary is definitely not the Mother of the Trinity
Rather, when we say that Mary is the Mother of God, we say that she is the Mother of God in the Incarnation of the Son of God. Remember, Jesus is One Divine Person taking up two natures, human and divine. What is the Biblical foundation of the Theotokos (God-bearer) who is Mary? Let us read Luke 1:41-43 and here's what it says:
Luke 1:41-43, "And when Elizabeth heard Mary's greeting, the infant in her womb, leaped for joy. And she was filled with the Holy Spirit and she cried out in a loud voice, Blessed are you among women, and blessed is the fruit of your womb! Who am I that the mother of my Lord should come to me?"
If we are going to do hermeneutics or studying the text in the culture of the time, we will see here that Elizabeth and Zechariah are both devout Jews (Luke 1:5-6). For them, mentioning the Name of God (Adonai/Yahweh/Elohim/HaShem) without reverence is blasphemy and is punishable by death through stoning (Exodus 20:7, cf. Leviticus 24:15-16). But here is Elizabeth, filled with the Holy Spirit and she even called her cousin, "the mother of my Lord". So literally speaking, Elizabeth is calling Mary, "The mother of my God". Remember, Lord and God can be interchangeable because the Lord (Adonai) can also mean God (Elohim). Here's proof from Mary's Magnificat and the book of Psalms, here's what it says:
Psalm 100:3, "Know therefore that the LORD is God, and that we are His. We are His people, the sheep of His flock"
Luke 1:46-47, "And Mary said, My soul magnifies the Lord, and my spirit rejoices in God, my Savior"
And the Torah itself says that the Lord God is one as Deuteronomy 6:4 says
Deuteronomy 6:4, "Hear O Israel, the LORD is our God, the LORD is one!"
Deuteronomy 6:4 (Hebrew phonetics), "Shema Yisrael, Adonai Eloheinu, Adonai echad!"
So, both the Lord of Elizabeth and Mary is God their Savior based on the passage. The doctrine of the Theotokos is the application of the Hypostatic Union. When Jesus was incarnated, His divinity was not lost. In fact, St. Paul testifies to this fact:
Colossians 2:9, "For in Him dwells the fullness of the Godhead bodily"
The visitation of Mary to Elizabeth
The fullness of the Godhead dwells in Jesus Christ
Where did this teaching of Mary as not the Mother of God originated? It originated from a priest called Nestorius. For him, he believed that Jesus Christ is two Persons and that Mary cannot be called the Mother of God, but of the human side of Jesus. This called for the Council of Ephesus wherein the Church formally declared Mary as the Mother of God since she gave birth to Jesus, one Divine Person who took on two natures, human and divine. Prior to the Council of Ephesus in 431 A.D., Christians already had a devotion to the Blessed Virgin Mary as the Mother of God. In the Sub Tuum Praesidium which was written in 250 A.D., here's what it says:
LATIN VERSION:Sub tuum praesidium confugimus,Sancta Dei Genetrix.Nostras deprecationes ne despicias in necessitatibus nostris,sed a periculis cunctis libera nos semper,Virgo gloriosa et benedicta
We fly to your patronage, O Holy Mother of God, despise not our petitions in our necessities, but ever deliver us from all dangers, O Glorious and Blessed Virgin
Usually, this prayer is said by some Catholics after the contemplation of the mysteries before they say the Salve Regina (Hail, Holy Queen).
Mothers give birth to persons, and not to natures, that we should know. For example, Mary in Acts 12 is NOT the mother of the body of John Mark, but she is the mother of John Mark. While the Church also accepts that Mary is the Mother of Jesus, Mother of Christ, we should also understand who Jesus really is. The early reformers like Martin Luther and John Calvin had reverence for Mary's divine maternity. Here are some of their quotes about Mary's Divine Motherhood:
"She is rightly called not only the mother of the man, but also the Mother of God ... It is certain that Mary is the Mother of the real and true God."- Martin Luther
"Elizabeth called Mary Mother of the Lord, because the unity of the person in the two natures of Christ was such that she could have said that the mortal man engendered in the womb of Mary was at the same time the eternal God."- John Calvin
We come to God through Mary. Martin Luther said so himself
Now, what can we learn from Mary's Divine Motherhood?
* We can become God bearers ourselves. We may not be relatives of our Lord by blood, but by being salt and light to others, they will see that we bear God in us. That is one of His commands in Matthew 5:16, "Let your light so shine before men, that they may see your good works and praise your Father who is in heaven"
To conclude this, I want to quote Luke 11:28, "And He said, Yes, but more than that, blessed is the one who hears the word of God and obey it"
Holy Mary, Mother of God, pray for us sinners, now and at the hour of our death. Amen.
"Queens and concubines sing your praises" (Song of Solomon 6:9)