On December 8, 1854 Pope Pius IX defined as an article of faith that “the most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.” To become the mother of the Savior, Mary "was enriched by God with gifts appropriate to such a role." (Catechism of the Catholic Church, 490)
It took the Church many centuries to come to this understanding of the singular grace that Mary had received at the moment of her conception.

Although the early Church’s Fathers spoke of Mary’s sinlessness and purity, it wasn’t until the 8^th century that the Eastern Church began to celebrate a Feast of the Conception of Mary; and in the 12^th century, the Church of the West followed. However, shortly after that, the doctrine of the Immaculate Conception hit a roadblock. When the feast of the [Immaculate] Conception of the Blessed Virgin Mary was introduced in France, St. Bernard of Clairvaux opposed it. Most Scholastic theologians, including St. Albert the Great, St. Thomas Aquinas, and St. Bonaventure also opposed it. The difficulty they had with celebrating Mary's Immaculate Conception was the conviction that every person is conceived with original sin. Mary had to be touched by original sin for at least an instant, they maintained. And Mary was either purified in the womb and, further purified at the time of the Annunciation. 

The doctrine of the Immaculate Conception was defended and explained with theological clarity in the thirteenth century by Blessed John Duns Scotus, a Franciscan. Duns Scotus explained that the Immaculate Conception came through God's application of the grace of Christ beforehand. He successfully argued that Christ can save and redeem in two ways: 
He can rescue from sin those already fallen; or He can preserve one from being touched by sin even for an instant. Mary was granted “redemption by exemption.” A few centuries later, in 1830, St. Catherine Labouré received a series of apparitions from Our Lady during which she received a medal that came to be known as the “Miraculous Medal.” Engraved on the medal were the words, “O Mary, conceived without sin, pray for us who have recourse to thee.”
The apparition of Mary Immaculate to St. Catherine Labouré advanced the devotion to the Immaculate Conception. And the solemn definition of the Dogma of the Immaculate Conception in 1854 by Pope Pius IX was the culmination of it. And like an additional seal of approval on the definition four years later, Mary appeared to a 14-year-old uneducated girl, Bernadette Soubirous, at Lourdes. When Bernadette asked the Virgin Mary on March 25, 1858, to identify herself, Mary replied, “I am the Immaculate Conception.”
The Solemnity of the Immaculate Conception is a reminder to all of us that the grace of God always precedes us and that with God nothing is impossible.
The same Christ by whose merits Mary was preserved from sharing in the universal human disaster, this same Christ makes Himself available to us today. The redemption that kept Mary sinless is still being offered to men and women of all walks of life and of all times. Let us surrender our lives daily to God. And let us ask for the intercession of the Immaculate Conception to cooperate with God’s grace in our daily lives.
 
“O Mary conceived without sin, pray for us who have recourse to you.”
 
Fr. Robain