Mary is often depicted in this way because her entire life has been one of victory over the devil. In the very beginning of the Bible, in Gen 3:15, God says to the serpent:
“I will put enmity between you and the woman, and between your seed and her seed; he shall bruise your head, and you shall bruise his heel.”
A bruise to the head of a serpent is fatal. So, God is saying that the seed, or offspring, of a woman will destroy the devil. In light of the New Testament, we know that this “he” who will bruise the head of the serpent is Jesus Christ. As the Letter to the Hebrews tells us:
“Since therefore the children share in flesh and blood, he himself likewise partook of the same nature, that through death he might destroy him who has the power of death, that is, the devil” (Heb 2:14).
What is interesting is that the Douay-Rheims translation (an older Catholic translation of the Bible from 1609 AD) says that “she” will bruise the head of the serpent, not “he”. This means that it is the woman that will destroy the devil, not her seed.
This is probably where our Marian art finds its inspiration. But, whether it’s a “he” or a “she,” the fact of Mary’s victory over the devil remains. Her “Yes” brought the Savior into the world, and her sinlessness only added insult to Satan’s injury.
We see Mary’s victory again at the very end of the Bible, in the 12th chapter of the Book of Revelation. The serpent, angry over the victory of the woman’s son, tries to defeat the woman. When that fails, he goes after her offspring, who are “those who keep the commandments of God and bear testimony to Jesus” (Rev 12:17).
That’s us! And, of course, we know that when we unite ourselves to Jesus, through Mary, the devil will fail in his pursuit of us as well. The image of Mary, with her heel pressed firmly on the head of the serpent, is a powerful image of the victory that that can be had through a total
self-surrender of our lives to the glory of God and the fulfillment of His will.