7 In the first place, the foundation of this doctrine is to be found in Sacred Scripture, where we are taught that God, Creator of all things, after the sad fall of Adam, addressed the serpent, the tempter and corrupter, in these words, which not a few Fathers, Doctors of the Church and many approved interpreters applied to the Virgin Mother of God: "I will put enmities between thee and the woman, and thy seed and her seed" ( Gen . III-15). Now, if at any time the Blessed Mary were destitute of Divine grace even for the briefest moment, because of contamination in her conception by the hereditary stain of sin, there would not have come between her and the serpent that perpetual enmity spoken of from earliest tradition down to the time of the solemn definition of the Immaculate Conception, but rather a certain subjection.
Source: Fulgens Corona (Vatican.va)