Roman Catholics believe in the Immaculate Conception of Mary, as proclaimed Ex Cathedra by Pope Pius IX in 1854, namely that she was filled with grace from the very moment of her conception in her mother's womb and preserved from the stain of original sin. The Latin Rite of the Roman Catholic Church has a liturgical feast by that name, kept on December 8. Orthodox Christians reject the Immaculate Conception dogma principally because their understanding of ancestral sin (the Greek term corresponding to the Latin "original sin") differs from the Augustinian interpretation and that of the Roman Catholic Church.
When did Pope Pius IX become a Roman Catholic?