Why the Immaculate Conception?

A First Things writer takes it on:

...The word “Immaculate” doesn’t simply mean “perfectly clean,” as we tend to think from its use in real estate ads, but “unstained.” The doctrine emphasizes Mary’s freedom from moral corruption—not, and this is the crucial point, what she is in herself but what she is by the grace of God. Issued by Pope Pius IX in the Apostolic Constitution Ineffabilis Deuson December 8, 1854, the definition declares 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.

She is, he wrote, “far above all the angels and all the saints so wondrously did God endow her with the abundance of all heavenly gifts poured from the treasury of his divinity.” Because God did this for her—because God did it—Mary, “ever absolutely free of all stain of sin, all fair and perfect, would possess that fullness of holy innocence and sanctity.”

Even very sympathetic Protestants think of it as a kind of devotional optional extra. But Pius thought it a very important doctrine to get right. Anyone who rejects it (he seems to be thinking only of Catholics here) is “condemned by his own judgment.” The dissenter should know “that he has suffered shipwreck in the faith; that he has separated from the unity of the Church.”

The pope explained it in terms of the fittingness that the Son of God should have such a mother, the Church’s liturgical practice in celebrating the Feast of the Conception of Mary, and the teaching and practice of previous popes, which he reviews at some length. He notes the agreement of religious orders, eminent theologians, and bishops, the “intimation” of the Council of Trent, and the testimony of “of venerable antiquity, of both the Eastern and the Western Church.” He then summarizes the biblical arguments offered by “the Fathers and writers of the Church” and their “explicit affirmation” of the doctrine.

Pius's argument, such as it is, does not satisfy Protestants, who ask, and quite rightly given their beliefs, “Just where is this in Scripture?” It looks to them as if the Catholic Church is rationalizing a doctrine that had grown too big to fail. They can understand how the Catholic might get from Jesus' statements at the Last Supper to a belief in Transubstantiation, but not how he can get from apparently no evidence whatsoever to the Immaculate Conception. That doesn't look like a stretch but an invention.

Yet, in Ineffabilis Deus itself, Pius said that the Church “never changes anything, never diminishes anything, never adds anything.” The Church, he would insist, is a witness, not an inventor, a reporter, not a novelist. And he is not wrong in saying so, though the reason gets at a deeper difference between the traditions than their beliefs about the Virgin Mary...

The whole article (and accompanying links) are well worth a read.