1854 AD
On December 8, 1854, Pope Pius IX defined the dogma of the Immaculate Conception by the apostolic constitution Ineffabilis Deus, declaring that 'the most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and in view of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.' This definition clarified a belief that had been held by the vast majority of Catholics for centuries but had not been formally defined as dogma.
The Immaculate Conception does not refer to the virgin birth of Jesus (a common misconception); rather, it refers to Mary's own conception in the womb of her mother, St. Anne. The dogma teaches that Mary was preserved from original sin from the moment of her conception, not that she was conceived without a father. This preservation was made possible by the anticipated merits of Jesus Christ, applied to Mary in a unique way.
The Catholic Church honors the Immaculate Conception as a solemn feast on December 8 and recognizes it as a unique privilege granted to Mary in preparation for her role as the Mother of God. Key Scripture: Luke 1:28, 42; Genesis 3:15; 2 Corinthians 5:21; CCC 490-493, 508-510