The Catechism of the Catholic Church (495) states the following:

Called in the Gospels “the mother of Jesus” (Jn 2:1), Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as “the Mother of my Lord” (Lk 1:43). In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father’s Eternal Son, the second Person of the Holy Trinity. Hence the Church confesses that Mary is truly “Mother of God” (Theotokos) (cf. the Council of Ephesus in 431 AD).

Three statements of Faith emphasize the teaching expounded in the Catholic Catechism quoted above, comprising a basic framework from which to defend the title given to Our Lady as the Mother of God:

Statement One: In this work whereby she was made the Mother of God, so many and such good things were given her that no one can grasp them. Not only was Mary the Mother of Him who is born [in Bethlehem], but of Him who, before the world, was eternally born of the Father, from a Mother in time and at the same time man and God.

Statement Two: It cannot be denied that God, in choosing and destining Mary to be the Mother of His son, granted her the highest honor… Elizabeth calls Mary the 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.

Statement Three: It was given to her what belongs to no creature, that in the flesh she should bring forth the Son of God.

No Medieval pope or council called Mary by the title of Mother of God, as cited in the Hail Mary. It was the Holy Spirit, 33 years before the foundation of the Church, who inspired Elizabeth to exclaim: “And how have I deserved that the mother of my Lord should come to me”? (Lk 1:43).

Who is this “Lord” St Elizabeth is talking about?

The Aramaic/Hebrew word for “my Lord” is Adonai. The Jews always used Adonai instead of the sacred name of God, which they had ceased to pronounce out of reverence. The Adonai, the “Lord,” that Elizabeth was talking about could not have been any adult lord of hers, such as the High Priest, the Roman Governor, or even her husband. They had already been born, and Mary was not expecting any of them—this is evident. She was the Mother of one Adonai as yet unborn, an unborn Lord.

The only one Lord, Adonai, who was going to be born was the Expected One of the nations: the Messias. And Elizabeth, inspired by the Holy Spirit who is God and can neither deceive nor be deceived, refers to the unborn Messias as Adonai, the same word used by Jewish prophets to refer to the Lord God of Hosts.

Who was the “Lord” of St Elizabeth, whose Mother Mary is? God is her Lord, her Adonai—and ours, too, for that matter.

That is why the Catholic Church could have chosen to say in the Hail Mary: Holy Mary, Mother of my Lord, pray for us sinners. Or Mother of my Adonai—it would have meant exactly the same thing as Holy Mary, Mother of God.

Moses cried out, “Hear, O Israel, the Lord our God is one Lord…” (Deut 6:4), or, “The Adonai our God is ONE Adonai…”!

There is only one God, in three distinct Persons. Mary conceived and gave birth to a divine Person; therefore, she is the Mother of that same divine Person—Mother of God.

On the day of the Annunciation and the Incarnation, the Archangel St Gabriel, “who stands in the presence of God” (Lk 1:19), also inspired by the Holy Spirit, said to Mary: “Fear not, Mary, for you have found grace with God. Behold, you shall conceive in your womb, and shall bring forth a son, and you shall call his name Jesus. He shall be great, and shall be called the Son of the Most High… The Holy One who shall be born of you shall be called the Son of God” (Lk 1:30,35).

Notice also how the Archangel said to Mary, “You shall call his name Jesus,” and adds three qualifications: “The son of the Most High … The Holy One … The Son of God.”

The title Holy One is often used by the Old Testament Prophets to refer to Almighty God, i.e.:

“I am the Lord, your Holy One, the creator of Israel, your King” (Is 43:15).

“I am God and not man, the Holy One present among you” (Hosea 11:9).

“I have not transgressed the commands of the Holy One” (Job 6:10).

Isaiah’s prophecy reads,“A virgin shall conceive and bear a son, and his name shall be called Emmanuel” (Is 7:14). In his Gospel, St Matthew interprets the name Emmanuel as meaning “God with us” (Mt 1:23), a divine Person living in the womb of His human Mother, then being born and living among us. That divine Person assumed that human nature from her.

The same Isaiah prophesied that the Son of the Virgin would be both human and divine: “For a child is born to us, and a son is given to us… and his name shall be called, Wonderful Counselor, Mighty God, Father-Forever, Prince of Peace” (Is 9:5).

Notice how Isaiah links together these words: child… son… Mighty God.

Can a woman be the mother of a divine Person?

A mother is a woman who conceives in her womb the body of her child and later gives birth to that same child. Everyone knows that. Yet she does not create the soul of the child—God creates it. But she bears both the body and the soul of the child in her womb: she bears the whole person.

No child can say to his mother, “You are only the mother of my body, not of my soul, because God created my soul, not you.”

That would be most disrespectful to his mother, and, besides, she did not create his body, either—in fact, she created nothing of him! She conceived his body, and God created his soul within his body at conception. She nurtured him and eventually gave birth to him. She is not only the mother of the constitutive natures of that being, that is, material body and spiritual soul: she is the mother of the whole person. This conclusion is most important: she is not only the mother of the child’s body and soul, but of the whole child, the person.

Jesus is one of the three divine Persons of the Holy Trinity—He is God. Mary is the Mother of Jesus. Mary is the Mother of one of the three divine Persons of the Holy Trinity. Mary is the Mother of God.

Now, with Jesus, God the Father replaces the natural father in the ordinary course of nature. Jesus’ Mother was overshadowed by the Holy Spirit and the Second Person of the Holy Trinity, God the Son, descended into her womb and assumed a human nature from her.

At the very moment of conception, God the Father created His human soul.

Therefore, His Mother bore His Body, His Soul and His Divinity, because there is only one Person in Jesus—a Divine Person, whom Isaias called the Mighty God and the Angel called the son of the Most High … the Holy One … the Son of God!

Mary is not, and could not possibly be, the creator of Jesus, since He is eternal, uncreated. She is His Mother because she bore Him in her womb, she nurtured Him there, and finally gave birth to Him. Any woman who does these things for a child is the mother of that child.

So, if the Mighty God, the Son of the Most High, the Holy One, is He who shares in the nature of His Father, it does not take much thinking to put two and two together: Since Jesus is God the Son, Mary is the Mother of Jesus, then Mary is the Mother of God the Son. Not the Mother of the Trinity, but the Mother of one of the Divine Persons—God the Son, who is just as much God as the other two Persons.

You cannot deny the divine Motherhood without denying the doctrine of the Holy Trinity.

 Early Christian and the divine Motherhood

The Early Christian writers believed and defended the very same teaching of the Catholic Catechism. By way of example:

Saint Ephrem, who died in the year 373 AD, wrote beautiful praises of the Virgin Mary, many of which are still used in the Syriac Liturgy to this day. One of them reads: In the womb of Mary the Infant was formed, Who from eternity is equal to the Father… And the handmaid and work of His wisdom became the Mother of God (Song of Praise, in The Faith of the Early Fathers, William Jurgens, vol. 1, p.312, The Liturgical Press, Collegeville, Minnesota,  1970).
Saint Irenaeus, the great defender of Christianity against heresies, upon writing against the Gnostics around the year 190 AD, said: The Virgin Mary … being obedient to God’s word, received from an angel the glad tidings that she should bear God (Adversus Haereses, ibid., p.101).
St Ignatius of  Antioch, one of the greatest luminaries of the post-Apostolic era, himself a martyr for the Faith, in a letter to the Christians in Ephesus (ca. 110 AD), stated quite clearly the Faith of the Church of Jesus Christ: Our God, Jesus Christ, was conceived by Mary in accord with God’s plan: of the seed of David, it is true, but also of the Holy Spirit (Letter to the Ephesians, ibid., p.18).

This is the Faith of the Early Christians.

The Councils of the Early Church

The bishops of the Council of Ephesus, in 431 AD, more than 1,000 years before the existence of Protestantism, unhesitatingly called the Holy Virgin “Mother of God (theotokos).” It was confirmed by the Council of Chalcedon twenty years later and remains to this day in the Catholic Church.

What did Luther, Calvin and Zwingli believe about Mary as Mother of God?

Martin Luther: In this work whereby she was made the Mother of God, so many and such good things were given her that no one can grasp them.  Not only was Mary the Mother of Him who is born [in Bethlehem], but of Him who, before the world, was eternally born of the Father, from a mother in time and at the same time man and God. (The Works of Luther, Weimar, English translation by Pelikan, Concordia, St Louis, Vol. 7, p.572)
John Calvin: It cannot be denied that God, in choosing and destining Mary to be the Mother of His son, granted her the highest honor…  Elizabeth calls Mary the 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. (Calvini Opera, Corpus Reformatorum, Braunschweig-Berlin, 1863-1900, vol. 45, pp. 348,35).
Ulrich Zwingli: It was given to her what belongs to no creature, that in the flesh she should bring forth the Son of God. (Zwingli Opera, Corpus Reformatorum, Berlin, 1905, in Evang. Luc., Op. comp., Vol. 6, I, p. 639).

Conclusion

Opposition to Mary’s title of Mother of God is unscriptural, illogical and unhistorical.

The early Christians—with the exception of a couple of heretics—believed in it quite naturally. It is unfortunate that separated brethren who thrive in opposing everything from the “R.C. Church” (as they term it) get so carried away that they end up opposing the Bible, the Faith of the early Christians and even the founders of Protestantism themselves.