From Luke 1:
In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, to a virgin betrothed to a man whose name was Joseph, of the house of David. And the virgin's name was Mary. And he came to her and said, “Greetings, O favored one, the Lord is with you!” But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be. And the angel said to her, “Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob forever, and of his kingdom there will be no end.”
And Mary said to the angel, “How will this be, since I am a virgin?”
And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God. And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren. For nothing will be impossible with God.” And Mary said, “Behold, I am the servant of the Lord; let it be to me according to your word.” And the angel departed from her.
The Catholic tradition has long recognized Mary's consent as the exemplar and epitome of faith. As the Catechism says, "The Virgin Mary most perfectly embodies the obedience of faith. By faith Mary welcomes the tidings and promise brought by the angel Gabriel, believing that 'with God nothing will be impossible' and so giving her assent: 'Behold I am the handmaid of the Lord; let it be [done] to me according to your word.'"
And also: "At the announcement that she would give birth to 'the Son of the Most High' without knowing man, by the power of the Holy Spirit, Mary responded with the obedience of faith, certain that 'with God nothing will be impossible': 'Behold, I am the handmaid of the Lord; let it be [done] to me according to your word.' Thus, giving her consent to God's word, Mary becomes the mother of Jesus.'"
Notice the words "assent" and "consent." In Mary we have in hand the biblical vision of "free will," our saying "yes" to God. And as I've also pointed out in this series, in regards to capacity, Mary can do nothing to make herself pregnant. But her assent and consent are necessary for the gift to be given.
God doesn't override or force the human will. God seeks our consent and participation with grace. In this, Mary is the example, model, and forerunner of us all.
