David’s Scott article on the dogma. He does an excellent job of setting the historical context. explaining the conflicts over the idea among theologians through history, and so on:

One hundred and fifty years later, the dogma of the Immaculate Conception can be understood even more clearly as the Church’s prophetic response to the modern world.

As Bishop Fulton Sheen pointed out near the 100th anniversary of the dogma: “The definition of the Immaculate Conception was made when the modern world was born.”

And it did come at a time when rival moral and spiritual blueprints were being drawn up for the modern world. Within five years, Marx would unveil his philosophy of atheist class struggle; Darwin would publish his theory of evolution; and John Stuart Mill, his ethics of radical individualism.

These architects of modernity shared an unshakable faith— that man had no need of God, that there was no such thing as original sin, and that human progress and perfection were not only possible but inevitable.

In other words, Bishop Sheen said, they believed that “everyone is immaculately conceived.”

From his exile, Blessed Pius seemed to sense all this.

And in defining Mary’s Immaculate Conception he was writing a new charter for the modern world. The dogma was a piece of resistance, a defiant vow to resist the false spirit of the emerging age.

The Immaculate Conception remains a powerful reminder of the reality of evil and of human sin. But it’s also a promise—that humanity is destined for something far greater.

Mary, the dogma revealed, was the clean slate humanity so desperately needed—the new Eden from which the world would be made new again.

Blessed Pius’ definition inspired a century of intense Marian piety and devotion—culminating with the declaration, in 1950, of the dogma of Mary’s final Assumption into heaven.

In defining the beginning and end of Mary’s life, the Church defined the meaning and destiny of every human life—to be transfigured by grace, freed from sin, made able to love with a pure heart, and to hope for heaven.

Mike Aquilina remarks:

In the East, Mary is all-holy, panagia. In the West, we celebrate her as sinless. These are two complementary aspects of the same truth. (And maybe the most perfect illustration of the difference between a “half full” and “half empty” approach to theology!)