It may be objected that a divine decree such as that of the Incarnation could not depend on the consent of a creature who was free not to give it. To this theology answers that God has efficaciously willed and infallibly foreseen everything that will happen in the course of time. Therefore, He willed efficaciously and foresaw infallibly Mary’s consent to the realization of the mystery of the Incarnation. From all eternity God, who works with strength and gentleness, decided to give Mary the efficacious grace which would move her to consent freely and meritoriously. Just as He makes the trees to bear their blossoms, so He makes our wills to produce their free acts; and far from doing them any violence He is the author of their freedom, for that too is a reality, a form of being. The “how” of all this is the secret of God Omnipotent. Just as Mary conceived the Saviour by the operation of the Holy Ghost without losing her virginity, so she uttered her fiat infallibly under the motion of efficacious grace without prejudice to her complete liberty—rather did her will, under the divine motion, flower spontaneously into the free consent she gave in the name of all mankind.

Garrigou-Lagrange O.P., Reverend Reginald. Reverend Reginald Garrigou-Lagrange O.P. Collection [16 Books] . Aeterna Press. Kindle Edition.