For the rest of humanity, the Church teaches that original sin was present in our souls, and later removed through the sacrament of baptism. Mary is unique in that she is conceived, not in a state of original sin, but rather is a state of original sanctity and innocence.
The subject of the conception of Mary and its proper interpretation has been discussed and debated virtually throughout the entire history of the Church. From as far back as the Desert Fathers, and later by such learned theologians as Origen, Tertullian, Ireneaus, St. Augustine, St, Ambrose, St. Bernard, St. Thomas Aquinas and Duns Scotus, opinions have been expressed and analyzed again and again and again. The Biblical foundation of this doctrine is often thought to be from Like’s Gospel (Lk 1:28), when Gabriel greets Mary with the words “Hail, Mary full of grace…”
Today, the Feast of the Immaculate Conception is celebrated as a holyday of obligation in the United States, and we Americans view ourselves as under the protection of our national patroness “Mary of the Immaculate Conception.”