On the Immaculate Conception

The Feast of the Immaculate Conception - December 8
(repost)
Short history

Doctrines are defined formally only when there is a controversy that needs to be cleared up or when by the magisterium the faithful can be helped by particular emphasis being drawn to some already-existing belief. The definition of the Immaculate Conception was prompted by the latter motive; it did not come about because there were widespread doubts about the doctrine.

It was clear in the first centuries that only a perfect holiness, including absence of sin, was fitting in view of the dignity of her role. By the 8th century belief that Mary’s holiness was both flawless and immense was firmly established throughout the East, and it was in that century that the feast of her conception was first celebrated liturgically. In the West the belief grew more slowly, but by 1099 St Anselm could write: “It was fitting that she be clothed with a purity so splendid that none greater under God could be conceived.”

The belief in the Immaculate Conception was initially met with theological difficulty. If Christ was the redeemer of all, as the Scriptures affirm, he would have been the redeemer of Mary too. But then how was it possible that she was conceived immaculate and therefore was not in need of redemption? St Thomas Aquinas, unable to resolve the difficulty, concluded that Mary was conceived in original sin but was cleansed from it before her birth.

The decisive argument came from the Franciscan John Duns Scotus (1264-1308), who reasoned that Mary too was in need of redemption, but she was redeemed from the moment of her conception through the merits of her Son’s death on the Cross many years later. Duns Scotus laid the foundations of the true doctrine so solidly and dispelled the objections in a manner so satisfactory, that from that time onward the doctrine prevailed.( more on Catholic Encyclopedia - Immaculate Conception) .

While theologians continued to debate the question for several more centuries, by the end of the 17th century there was practically universal agreement on Mary’s immaculate conception. The doctrine of the Immaculate Conception was officially defined by Pope Pius IX in 1854. When Fundamentalists claim that the doctrine was "invented" at this time, they misunderstand both the history of dogmas and what prompts the Church to issue, from time to time, definitive pronouncements regarding faith or morals.

The dogma

In the Encyclical Ineffabilis Deus by Pope Pius IX:

We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.

...Under her guidance, under her patronage, under her kindness and protection, nothing is to be feared; nothing is hopeless. Because, while bearing toward us a truly motherly affection and having in her care the work of our salvation, she is solicitous about the whole human race. And since she has been appointed by God to be the Queen of heaven and earth, and is exalted above all the choirs of angels and saints, and even stands at the right hand of her only-begotten Son, Jesus Christ our Lord, she presents our petitions in a most efficacious manner. What she asks, she obtains. Her pleas can never be unheard.
/
Update: Read Mark Shea's excellent post on the development of the Immaculate Conception dogma.