Mary attains a union with God that exceeds all the expectations of the human spirit. It even exceeds the expectations of all Israel, in particular the daughters of this Chosen People, who, on the basis of the promise, could hope that one of their number would one day become the mother of the Messiah. Who among them, however, could have imagined that the promised Messiah would be “the Son of the Most High”? On the basis of the Old Testament’s monotheistic faith such a thing was difficult to imagine. Only by the power of the Holy Spirit, who “overshadowed” her, was Mary able to accept what is “impossible with men, but not with God” (cf. Mk 10: 27).
Thus the “fullness of time” manifests the extraordinary dignity of the “woman”. On the one hand, this dignity consists in the supernatural elevation to union with God in Jesus Christ, which determines the ultimate finality of the existence of every person both on earth and in eternity. From this point of view, the “woman” is the representative and the archetype of the whole human race: she represents the humanity which belongs to all human beings, both men and women. On the other hand, however, the event at Nazareth highlights a form of union with the living God which can only belong to the “woman”, Mary: the union between mother and son. The Virgin of Nazareth truly becomes the Mother of God.
This truth, which Christian faith has accepted from the beginning, was solemnly defined at the Council of Ephesus (431 A.D.). In opposition to the opinion of Nestorius, who held that Mary was only the mother of the man Jesus, this Council emphasized the essential meaning of the motherhood of the Virgin Mary. At the moment of the Annunciation, by responding with her “fiat”, Mary conceived a man who was the Son of God, of one substance with the Father. Therefore she is truly the Mother of God, because motherhood concerns the whole person, not just the body, nor even just human “nature”. In this way the name Theotókos – Mother of God – became the name proper to the union with God granted to the Virgin Mary.
The particular union of the Theotókos with God – which fulfils in the most eminent manner the supernatural predestination to union with the Father which is granted to every human being (filii in Filio) – is a pure grace and, as such, a gift of the Spirit. At the same time, however, through her response of faith Mary exercises her free will and thus fully shares with her personal and feminine “I” in the event of the Incarnation. With her fiat, Mary becomes the authentic subject of that union with God which was realized in the mystery of the Incarnation of the Word, who is of one substance with the Father. All of God’s action in human history at all times respects the free will of the human “I”. And such was the case with the Annunciation at Nazareth. – from Mulieris Dignitatem (1988)