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, 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. "The Son of God...born of the Virgin Mary...has truly been made one of us," has 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 cancelling out that nature.
Source: Redemptoris Mater (Vatican.va)