What God says, He does. The exceptional grace that God gave to Mary was the ability to say a perfectly free yes when He asked her to be the Mother of the Savior.
On her own Mary could not have escaped the mark of original sin. But to God nothing is impossible. Mary is filled with grace; the Immaculate Conception is an absolutely free gift from God. This is what Catholics believe. Protestants have no difficulty either in believing in God's free gifts. They should stop listening to those who like to assert that Catholics and Orthodox look at Mary as if she were God.
No Catholic or Orthodox believes this. They simply believe that Mary is All Pure, All Immaculate, and has been so from the moment of her conception, because this privilege was a gift from God. And if by any chance someone believes that this is impossible, they are diminishing the power of God’s grace.
The evangelist Saint Luke, inspired by the Holy Spirit, immortalized the words of the Angel Kékaritomene, "filled with grace." And we read again, in the song of Mary—the Magnificat—when she was greeted by Elizabeth: "The Mighty One has done great things for me" and “all generations will call me blessed."
Some non-Catholic authors retort that Saint Augustine (in the 5th century, or Saint Thomas Aquinas in the 13th century) thought that the Virgin Mary could not have been exempt from original sin before the coming Christ. In truth, this is not an easy question. God allows that even the most learned Doctors of the Church do not always have a perfect intellectual grasp on everything. But the Holy Spirit, who has been assisting the Church since Pentecost and leading her to the whole truth (Saint John), has inspired the Church to interpret Scripture and God's Revelation.
Until the Church makes a solemn pronouncement, theologians and Church leaders are free to hold different opinions. But once the Church has spoken*, with the assistance of the Holy Spirit, it is no longer possible to interpret Scripture differently. Whereas the humble receive God’s light by trusting in the Church assisted by the Holy Spirit, for the proud it represents a stumbling block.
*The Immaculate Conception is a dogma of the Catholic Church which states that the Virgin Mary was free of original sin from the moment of her conception. Pope Pius IX promulgated Ineffabilis Deus and adopted it as Church dogma in 1854. In 1858, Our Lady of Lourdes said to Saint Bernadette: "I am the Immaculate Conception."
Adapted from an article by Herve Marie Catta