Pius IX declaring the dogma of the Immaculate Conception.
Although the belief that Mary was sinless, or conceived with an immaculate soul, has been widely held since Late Antiquity, the doctrine was not dogmatically defined until 1854, by Pope Pius IX in his papal bull Ineffabilis Deus. The Catholic Church celebrates the Feast of the Immaculate Conception on December 8; in many Catholic countries, it is a holy day of obligation.
PIUS IX'S DECLARATION:
'We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful'. — Pope Pius IX, Ineffabilis Deus, December 8, 1854
The Immaculate Conception is the conception of the Blessed Virgin Mary free from original sin by virtue of the foreseen merits of her son Jesus Christ. The Church teaches that Mary was conceived by normal biological means in the womb of her mother, Saint Anne, but God acted upon her soul, keeping it "immaculate".