The great attack on the Virgin Mary, for some, concerns her Immaculate Conception, or the fact that she was preserved from original sin. Those who contest this point think they are causing a problem for Catholics alone. They do not realize that the Orthodox believe the exact same thing, under a slightly different name-they say she is All-Pure. Yet, even for a Protestant, the Immaculate Conception should be something completely in line with the Scriptures.
Before Original Sin, Eve too was without sin
Clearly, Mary did not make herself exempt from original sin, it was a gift from God. "The Almighty has done great wonders for me," said Mary in the canticle of the Magnificat (Luke 1: 49). And why would it be impossible for God to do for Mary what he had done for Eve? For before the fall, Eve was without original sin! Who are we to tell God what he should do or not do?
Scripture reveals the reality of this gift of God to the Virgin Mary in many places. First from the mouth of the Archangel Gabriel who says to Mary rejoice or hail, because in Greek the greeting was expressed by the word kaire = rejoice. A moment later Gabriel says kekaritomene, "you have been filled with grace." What does this mean? We do not know of any other passage in the whole Bible where an angel of God greets a man or a woman this way. So, do we need to take it as an empty formula in which the Angel just meant to say to Mary that she was "very lucky"? Obviously not.
The exceptional grace which God gave Mary was the capacity to answer with a perfectly free "yes"
The words of the Angel have a deep and real meaning. God said, and he did. The exceptional grace which God gave Mary was the capacity to answer God's request with a perfectly free "yes" to become the mother of the Savior. By herself Mary wouldn't have been able to escape the mark of original sin. But for God all things are possible. Mary is full of grace. The Immaculate Conception is God's absolutely free gift. This is what Catholics believe. And Protestants have no difficulty either in believing in the free gift of God. They should just stop imagining that Catholics (and Orthodox) believe that the Virgin Mary is God.
Mary is All-Pure, from the moment of her conception, because it was God's gift
Of course no Catholic or Orthodox believes that Mary is God: they simply believe that Mary is All-Pure, wholly Immaculate, from her conception, because it was God's gift. And if by chance someone thinks that it isn't possible, that person reduces God's gift. The Evangelist Saint Luke, inspired by the Holy Spirit, reported to us the word of the Angel kekarilomene, "full of grace." And we hear Mary's singing the words before Elizabeth in her Magnificat:
"The Almighty has done great wonders for me and all generations will call me blessed."
To trouble simple minds, some non-Catholic authors have tried to use the opinions of Saint Augustine (5th C) or Saint Thomas Aquinas (13th C), who thought that the Virgin Mary could not have been preserved from original sin before Christ. Granted, that was a difficult question, and God has allowed that even the most learned Doctors of the Church would not have a perfect reasoning over each and every question.
Fortunately the Holy Spirit, who has guided the Church since the day of Pentecost, and leads her to the whole truth (cf. John) has inspired the Church to interpret Scripture and God's Revelation as long as the Church has not pronounced herself, the Doctors and pastors had a right to offer their different opinions. But once the Church has ruled, with the assistance of the Holy Spirit, it is no longer possible to interpret the Scriptures in a different way.
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(Excerpt from a talk about Mary and Protestants)