Today the Church celebrates the Solemnity of Mary, Mother of God. This teaching is one of the four Marian Doctrines of the Catholic Church. The Catechism of the Catholic Church confirms the teaching by instructing, "that Mary is truly 'Mother of God' (Theotokos)." (CCC 495)
Doctrine Declared in 431
In 431, the Council of Ephesus confirmed Mary as Mother of God, Theotokos. This was proclaimed to answer a heresy denying Jesus’ dual nature as both human and divine. The Council declared:
"We confess, then, our Lord Jesus Christ, the only begotten Son of God, perfect God and perfect man, of a rational soul and a body, begotten before all ages from the Father in his Godhead, the same in the last days, for us and for our salvation, born of Mary the Virgin according to his humanity, one and the same consubstantial with the Father in Godhead and consubstantial with us in humanity, for a union of two natures took place. Therefore we confess one Christ, one Son, one Lord. According to this understanding of the unconfused union, we confess the holy Virgin to be the Mother of God because God the Word took flesh and became man and from his very conception united to himself the temple he took from her" (Formula of Union [A.D. 431]).Source of All Mary's Perfection
St. Josemaria Escriva examined the Mary's motherhood as the source of all her perfection, privileges and standing:
The Blessed Trinity, in choosing Mary as the Mother of Christ, a Man like us, has brought each one of us under the shelter of her maternal cloak. She is the Mother of God and our Mother.
The divine Motherhood of Mary is the source of all the perfections and privileges with which she is endowed. Because of it, she was conceived immaculate and is full of grace; because of it, she is ever virgin, she was taken up body and soul to heaven and has been crowned Queen of all creation, above the angels and saints. Greater than she, none but God. ‘The Blessed Virgin from the fact that she is the Mother of God has a certain infinite dignity which comes from the infinite good, which is God.’ There is no danger of exaggerating. We can never hope to fathom this inexpressible mystery; nor will we ever be able to give sufficient thanks to our Mother for bringing us into such intimacy with the Blessed Trinity.
We were sinners and enemies of God. Redemption has not only freed us from sin and reconciled us with Our Lord. It has also made us into children of God and has given us a Mother, the very Mother who gave birth to the Word when he took human nature. Could there ever be a greater, more generous, outpouring of love? (Friends of God, 276)Understanding of the Doctrine Expanded
Pope John Paul II explained the doctrine within the context of Vatican II and then expanded the Church's understanding of the doctrine in his 1987 encyclical, Redemptoris Mater:
4. The Second Vatican Council prepares us for this by presenting in its teaching the Mother of God in the mystery of Christ and of the Church. If it is true, as the Council itself proclaims,8 that "only in the mystery of the Incarnate Word does the mystery of man take on light," then this principle must be applied in a very particular way to that exceptional "daughter of the human race," that extraordinary "woman" who became the Mother of Christ. Only in the mystery of Christ is her mystery fully made clear. Thus has the Church sought to interpret it from the very beginning: the mystery of the Incarnation has enabled her to penetrate and to make ever clearer the mystery of the Mother of the Incarnate Word. The Council of Ephesus (431) was of decisive importance in clarifying this, for during that Council, to the great joy of Christians, the truth of the divine motherhood of Mary was solemnly confirmed as a truth of the Church's faith. Mary is the Mother of God (= Theotókos), since by the power of the Holy Spirit she conceived in her virginal womb and brought into the world Jesus Christ, the Son of God, who is of one being with the Father.9 "The Son of God...born of the Virgin Mary...has truly been made one of us,"10has been made man. Thus, through the mystery of Christ, on the horizon of the Church's faith there shines in its fullness the mystery of his Mother. In turn, the dogma of the divine motherhood of Mary was for the Council of Ephesus and is for the Church like a seal upon the dogma of the Incarnation, in which the Word truly assumes human nature into the unity of his person, without canceling out that nature.
5. The Second Vatican Council, by presenting Mary in the mystery of Christ, also finds the path to a deeper understanding of the mystery of the Church. Mary, as the Mother of Christ, is in a particular way united with the Church, "which the Lord established as his own body."11 It is significant that the conciliar text places this truth about the Church as the Body of Christ (according to the teaching of the Pauline Letters) in close proximity to the truth that the Son of God "through the power of the Holy Spirit was born of the Virgin Mary." The reality of the Incarnation finds a sort of extension in the mystery of the Church-the Body of Christ. And one cannot think of the reality of the Incarnation without referring to Mary, the Mother of the Incarnate Word.Behold Your Mother
Mary is our Holy Mother too. Although He was on the verge of death, Christ called out to John from the cross, "Behold your mother!" (John 19:27). Furthermore, since Mary is the mother of Christ, who is head of the Church, she is also Mother of the Church.
I will conclude with this simple, logical syllogism: Jesus is God. Mary is the mother of Jesus. Therefore, Mary is the mother of God.