The Virgin, absent original sin which would have turned her back on herself, hears the Word of God and does it.” (Lk. 11, 28). The Virgin is actively receptive of the Word that is spoken to her by the Spirit. By receiving Him in her spirit, she engenders Him in her flesh. His flesh, the flesh of God Himself, is totally taken from her. He is really her Son. And if there was the slightest holding back in giving Him her total humanity, He, God though He was, would not have been completely man. A most profound theologian wrote: “Abraham himself had been asked for a measureless obedience of faith: on Mount Moriah he was required to return the very gift that God had given him in exchange for his faith, namely the son of the promise. Abraham had performed the sacrifice spiritually, even if he had not had to carry it out materially. In Mary’s case, God goes to the end of this faith: on the Cross, under which she stands, no angel intervenes to save, and she must give back to God her Son, the Son of the fulfillment, in darkness of faith that she cannot comprehend or penetrate.
“But Jesus’ conception itself requires an act of faith that infinitely transcends that of Abraham (and especially that of Sarah, who laughed in her unbelief). The Word of God who wills to take flesh in Mary needs a receptive Yes that is spoken with the whole person, spirit and body, with absolutely no (even unconscious) restrictions, that offers the entirety of human nature as a locus for the Incarnation. Receiving and letting in need not be passive; in relation to God, they are, when done in faith, always supreme activity. If Mary’s Yes had contained even the shadow of a demurral, of a ‘so far and no farther,’ a stain would have clung to her faith and the child could not have taken possession of the whole of human nature. The freedom of Mary’s Yes from all hesitation comes perhaps most clearly to light where she also says Yes to her marriage with Joseph and leaves it to God to reconcile it with her new task.”
What is true of the Virgin is true of us: “whoever does the will of God is my brother and sister and mother.’ Mark 3:35. If we permit the Love Who is the Spirit to work in us to make the gift of ourselves, we will – like the Mother – engender Jesus Christ in us.
 Hans Urs von Balthasar, Mary the Church at the Source, Ignatius (2005) 104.