The Scriptural foundations for the Dogma of the Immaculate Conception are found in two key places in the Bible. In the OT with Genesis 3:15 and in the NT with Luke 1:28.
In Genesis 3:15 (also known as the protoevangelium, “the first Gospel”), God curses the serpent and says that He will put enmity between the serpent and the woman and between her seed and the serpents seed. We know from revelation that the seed of victory is none other than Jesus Christ, so the mother of the seed of victory can only be Mary. The word “enmity” means “a complete and total opposition.” So just as Christ is in total opposition to sin and Satan, likewise is Mary. In order for Mary to be in total and complete opposition to Satan, she would have to be conceived Immaculate, without the stain of original sin. Otherwise, Mary, had she been born with original sin, would not be at enmity with Satan. She would have to be playing for both sides. She would be tainted by Satan, but also working for God. In the words of Dr. Miravalle, “Mary is not a double agent!” As Genesis 3:15 states, she is in complete opposition to Satan and thus not corrupted by original sin.
In Luke 1:28, at the Annunciation, the Angel Gabriel greets Mary with the words, “Hail, Full of Grace! The Lord is with thee.” “Full of Grace” is Mary’s title. The Angel doesn’t say, “Hail Mary, Full of Grace,” but “Hail, Full of Grace.” The Angel testifies that Mary possesses a plenitude of grace and that the Lord is with her! The Angel’s testimony is so true indeed, he gives her a new name: Full of Grace! She would not have a plenitude of Grace had she been born with original sin.
In the Greek, the phrase is in the passive tense and would be literally translated as “Hail, You who have been fully graced!” This implies that Mary has received a plenitude of grace prior to the Annunciation and that the grace is ongoing. If there was any sin in the Blessed Virgin, whether original or any thereafter, the greeting of the Angel would be false! (And since, Scripture is the Inspired Word of God, we know that it is not.) Thus, Mary had to have been conceived Immaculately without the stain of original sin…AND SHE WAS!