Mary was saved by her Immaculate Conception in the womb of her mother. All of humanity was saved on Good Friday. It is part of the universal salvific will of God. How it is applied to each individual soul depends on the way that soul cooperates with the will of God and His graces that He bestows upon it.
Mary, who is part of the human race, would have been born with original sin. She would have suffered the consequences of original sin had it not been for the miraculous event of God in which He prefigured His Son’s redemption and applied it to the soul of Mary at the very moment of her conception. This is how Mary was saved.
Catholics and non-Catholic Christians alike are saved in the waters of baptism. Baptism removes the stain of original sin. The outward sign of baptism is water, the physical vehicle that transmits the blood of Christ shed on the cross to the soul. Once the negative is removed, there is room for the positive—grace. Supernatural grace is God’s life within the soul, also known as sanctifying grace. The only way sanctifying grace is lost is by personal sin that is mortal. It is restored by the Sacrament of Penance, which applies the blood of Christ through the words of absolution.
For Mary, all this took place immediately upon conception, hence the title Immaculate Conception. It is a doctrine of the Church that when Mary reached the age of reason, she never committed personal sin. So attuned to the life of grace, so close to Almighty God was she, that she never wanted to lose sanctifying grace. As the Archangel Gabriel greeted her when he announced that she would be the mother of God, “Hail Mary full of grace.” Saint Therese of Liseux once explained this miraculous event by stating, “Mary’s soul was like a full glass of water that has no room for anything else.” Filled with God’s grace, there was no room for sin, original or actual. She could then give her son, Jesus, an untainted and spotless human nature.
Saint John the Baptist was possibly the only other human born without original sin; however, he was conceived in the normal fashion every human is. This means that by being human, he inherited original sin upon conception. Scripture tells us that when Mary, who was pregnant with Jesus, visited Saint Elizabeth, who was pregnant with Saint John the Baptist, the baby in Saint Elizabeth’s womb leapt for joy. Many theologians contend that this is when Saint John was, in effect, “baptized.” God the Father took the saving events of His Son who was in the womb of Mary and applied it to Saint John the Baptist.