In Redemptoris Mater Pope John Paul II speaks to us about Mary's openness to God.

The word of the living God, announced to Mary by the angle, referred to her: “And behold, you will conceive in your womb and bear a son” (Luke 1:31). By accepting this announcement, Mary was to become “Mother of the Lord,” and the divine mystery of the Incarnation was to be accomplished in her: “The Father of mercies willed that the consent of the predestined Mother should precede the Incarnation.” And Mary gives this consent, after she has heard everything the messenger has to say. She says: “Behold, I am the handmaid of the Lord; let it be done to me according to your word” (Luke 1:38). This fiat of Mary—“let it be to me”—was decisive, on the human level, for the accomplishment of the divine mystery. There is a complete harmony with the words of the Son, who, according to the Letter to the Hebrews, says to the Father as he comes into the world: “Sacrifices and offering you have not desired, but a body you have prepared for me …. Lo, I have come to do your will, O God” (Hebrews 10:5-7). The mystery of the Incarnation was accomplished when Mary uttered her fiat: “ Let it be to me according to your word,” which made possible, as far as it depended upon her in the divine plan, the granting of her Son's desire. (13.3)