Catholic devotion to the idea that Mary was conceived without the stain of original sin had circulated for centuries before Pius IX, argued over by theologians and celebrated in popular feast days without ever being formally defined as required belief. In 1854, after consulting the bishops of the world by letter and receiving overwhelming support, Pius IX issued the bull Ineffabilis Deus, declaring the Immaculate Conception a dogma that all Catholics were bound to hold, using his authority as pope to settle a question theologians had debated since the Middle Ages without waiting for a council to do so.
The declaration mattered beyond its specific content because of how it was made. Pius IX did not summon bishops to debate and vote on the question in the manner of the ancient councils; he consulted them individually and then proclaimed the doctrine himself, treating papal authority as sufficient on its own to settle a matter of faith. This method anticipated, and to some historians directly prepared the ground for, the doctrine of papal infallibility that the First Vatican Council would formally define sixteen years later, since Pius had, in practice, already exercised the kind of authority infallibility would describe.
The dogma also connected directly to a wave of Marian apparitions reported during his pontificate, most famously at Lourdes in 1858, only four years after Ineffabilis Deus, where the visionary Bernadette Soubirous reported the figure identifying herself using words Bernadette, untrained in theology, would not otherwise have known, I am the Immaculate Conception, a detail widely read at the time as confirming the pope's recent definition from outside the church's own hierarchy.