Mary's most fundamental privilege is that of being the Mother of God. Pius XI quoted St. Thomas Aquinas with approval in saying that "From the fact that she is the Mother of God, she has a sort of infinite dignity from the infinite good that God is. (Lux veritatis, Dec. 25, 1931, citing St. Thomas, Summa Theologiae I. 25. 6. ad 4).

                                                                                                        ~ ( Father William G. Most)

A woman is a man’s mother either if she carried him in her womb or if she was the woman contributing half of his genetic matter or both. Mary was the mother of Jesus in both of these senses; because she not only carried Jesus in her womb but also supplied all of the genetic matter for his human body, since it was through her—not Joseph—that Jesus "was descended from David according to the flesh" (Rom. 1:3).

Since Mary is Jesus’ mother, it must be concluded that she is also the Mother of God: If Mary is the mother of Jesus, and if Jesus is God, then Mary is the Mother of God. There is no way out of this logical syllogism, the valid form of which has been recognized by classical logicians since before the time of Christ.

Although Mary is the Mother of God, she is not his mother in the sense that she is older than God or the source of her Son’s divinity, for she is neither. Rather, we say that she is the Mother of God in the sense that she carried in her womb a divine person—Jesus Christ, God "in the flesh" (2 John 7, cf. John 1:14)—and in the sense that she contributed the genetic matter to the human form God took in Jesus Christ.

To avoid this conclusion, Fundamentalists often assert that Mary did not carry God in her womb, but only carried Christ’s human nature. This assertion reinvents a heresy from the fifth century known as Nestorianism, which runs aground on the fact that a mother does not merely carry the human nature of her child in her womb. Rather, she carries the person of her child. Women do not give birth to human natures; they give birth to persons. Mary thus carried and gave birth to the person of Jesus Christ, and the person she gave birth to was God.

The Nestorian claim that Mary did not give birth to the unified person of Jesus Christ attempts to separate Christ’s human nature from his divine nature, creating two separate and distinct persons—one divine and one human—united in a loose affiliation. It is therefore a Christological heresy, which even the Protestant Reformers recognized. Both Martin Luther and John Calvin insisted on Mary’s divine maternity. In fact, it even appears that Nestorius himself may not have believed the heresy named after him. Further, the "Nestorian" church has now signed a joint declaration on Christology with the Catholic Church and recognizes Mary’s divine maternity, just as other Christians do.

Since denying that Mary is God’s mother implies doubt about Jesus’ divinity, it is clear why Christians (until recent times) have been unanimous in proclaiming Mary as Mother of God.