The Annunciation to Mary, according to the Scriptures, inaugurates "the fullness of time", the time of the fulfilment of God's promises and preparations. The Church teaches that Mary was invited to conceive him in whom the "whole fullness of deity" would dwell "bodily". The divine response to her question, "How can this be, since I know not man?", was given by the power of the Spirit: "The Holy Spirit will come upon you."
The mission of the Holy Spirit is always conjoined and ordered to that of the Son. The Catholic Church believes that the Holy Spirit, "the Lord, the giver of Life", was sent to sanctify the womb of the Virgin Mary and divinely impregnate it, causing her to conceive the eternal Son of the Father in a humanity drawn from her own.
The Church teaches that the Father's only Son, conceived as man in the womb of the Virgin Mary, is Christ. That is to say, anointed by the Holy Spirit, from the beginning of his human existence, though the manifestation of this fact takes place only progressively: to the shepherds, to the magi, to John the Baptist, and to the disciples. Thus the whole life of Jesus Christ will make manifest "how God anointed Jesus of Nazareth with the Holy Spirit and with power". (Acts 10:38)
What the Catholic faith believes about Mary is based on what it believes about Christ, and what it teaches about Mary illumines in turn its faith in Christ.
According to Church tutelage, "God sent forth his Son", but to prepare a body for him, he wanted the free co-operation of a creature. For this, from all eternity God chose for the mother of his Son a daughter of Israel, a young Jewish woman of Nazareth in Galilee, "a virgin betrothed to a man whose name was Joseph, of the house of David; and the virgin's name was Mary." (Lk 1:26-27).
Lumen Gentium
The dogmatic constitution on the Church, Lumen Gentium states: "The Father of mercies willed that the Incarnation should be preceded by assent on the part of the predestined mother, so that just as a woman had a share in the coming of death, so also should a woman contribute to the coming of life."
Throughout the Old Covenant the mission of many holy women prepared for that of Mary. At the very beginning there was Eve; despite her disobedience, she receives the promise of a posterity that will be victorious over evil, as well as the promise that she will be the mother of all the living. By virtue of this promise, Sarah conceives a son in spite of her old age. Against all human expectation God chooses those who were considered powerless and weak to show his faithfulness to his promises: Hannah, the mother of Samuel; Deborah; Ruth; Judith and Esther; and many other women.
As it states in Lumen Gentium: Mary "stands out among the poor and humble of the Lord, who confidently hope for and receive salvation from him. After a long period of waiting the times are fulfilled in her, the exalted Daughter of Sion, and the new plan of salvation is established."