13 As the Council teaches, "'The obedience of faith' (Rom. 16:26; cf. Rom. 1:5; 2 Cor. 10:5-6) must be given to God who reveals, an obedience by which man entrusts his whole self freely to God." This description of faith found perfect realization in Mary. The "decisive" moment was the Annunciation, and the very words of Elizabeth: "And blessed is she who believed" refer primarily to that very moment. Indeed, at the Annunciation Mary entrusted herself to God completely, with the "full submission of intellect and will," manifesting "the obedience of faith" to him who spoke to her through his messenger. She responded, therefore, with all her human and feminine "I," and this response of faith included both perfect cooperation with "the grace of God that precedes and assists" and perfect openness to the action of the Holy Spirit, who "constantly brings faith to completion by his gifts." The word of the living God, announced to Mary by the angel, referred to her: "And behold, you will conceive in your womb and bear a son" (Lk. 1:31). By accepting this announcement, Mary was to become the "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 to me according to your word" (Lk. 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" (Heb. 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. Mary uttered this fiat in faith. In faith she entrusted herself to God without reserve and "devoted herself totally as the handmaid of the Lord to the person and work of her Son." And as the Fathers of the Church teach-she conceived this Son in her mind before she conceived him in her womb: precisely in faith! Rightly therefore does Elizabeth praise Mary: "And blessed is she who believed that there would be a fulfillment of what was spoken to her from the Lord." These words have already been fulfilled: Mary of Nazareth presents herself at the threshold of Elizabeth and Zechariah's house as the Mother of the Son of God. This is Elizabeth's joyful discovery: "The mother of my Lord comes to me"!
Source: Redemptoris Mater (Vatican.va)