The Immaculate Conception is Mary's singular title affirming that she was free from original sin and its effects from the moment she was conceived in St. Anne's womb. She alone was never without the sanctifying presence of God in Her soul. Her passions, therefore, were not stained with disorder. Her will was not tainted with malice and inclined towards evil. Her intellect was not dimmed nor sluggish. Neither did Her body have the defects of fallen human nature and the suffering and death that accompanies such defects.
Pope Pius IX in his 1854 papal bull Ineffabilis Deus infallibly defined the dogma:
We declare, pronounce and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of Her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.
The "Mother of God" was already called "Immaculate Mary" in A.D. 649 by the Lateran Council. In his encyclical, Pius IX noted that previous popes had taught this same doctrine. He quoted Pope Alexander VII from his 1661 apostolic constitution Sollicitudo Omnium Ecclesiarum:
Concerning the most Blessed Virgin Mary, Mother of God, ancient indeed is that devotion of the faithful based on the belief that her soul, in the first instant of its creation ... was, by a special grace and privilege of God, in view of the merits of Jesus Christ, Her Son and the Redeemer of the human race, preserved free from all stain of original sin.
During Her apparitions at Lourdes in 1858, Mary affirmed, "I am the Immaculate Conception." Eight years prior to Pius IX's formal declaration, U.S. bishops in 1846 chose Mary as their patroness under the title "Blessed Virgin Mary, conceived without sin."
Learn why Our Lady is so profoundly blessed in season one of Church Militant's Premium show The One True Faith—Mary, Mother of the Church.