When did Pope Pius IX declare the Immaculate Conception a Roman Catholic dogma?

Pope Pius IX formally declared the Immaculate Conception a Roman Catholic dogma on December 8, 1854, through the papal bull Ineffabilis Deus.123

While this declaration occurred in the 19th century, the theological foundation for the dogma was significantly advanced centuries earlier by the Scottish philosopher and theologian Blessed John Duns Scotus. He provided a classical defense of the doctrine, arguing that God applied Christ’s redemptive grace to Mary at the moment of her conception to preserve her from original sin.43