ANSWER: Consent!


Mary’s consent to the angel’s message was necessary for the Redemption of mankind.

According to the 1917 Catholic Encyclopedia, many Church Fathers  - including St. Augustine, St. Jerome, St. Cyril and Ephrem – “say that the consent of Mary was essential to the redemption. It was the will of God, St. Thomas says (Summa III:30), that the redemption of mankind should depend upon the consent of the Virgin Mary. This does not mean that God in His plans was bound by the will of a creature, and that man would not have been redeemed, if Mary had not consented. It only means that the consent of Mary was foreseen from all eternity, and therefore was received as essential into the design of God.” (Source: Catholic Encyclopedia article on the Annunciation)