In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced and defined that the Blessed Virgin Mary "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin."

The subject of this immunity from original sin is the person of Mary at the moment of the creation of her soul and its infusion into her body. The formal active essence of original sin was not removed from her soul, as it is removed from others by baptism; it was excluded, it never was in her soul. Simultaneously with the exclusion of sin. The state of original sanctity, innocence, and justice, as opposed to original sin, was conferred upon her, by which gift every stain and fault, all depraved emotions, passions, and debilities, essentially pertaining to original sin, were excluded. But she was not made exempt from the temporal penalties of Adam — from sorrow, bodily infirmities, and death.

The immunity from original sin was given to Mary by a singular exemption from a universal law through the same merits of Christ, by which other men are cleansed from sin by baptism. Mary needed the redeeming Saviour to obtain this exemption, and to be delivered from the universal necessity and debt (debitum) of being subject to original sin. The person of Mary, in consequence of her origin from Adam, should have been subject to sin, but, being the new Eve who was to be the mother of the new Adam, she was, by the eternal counsel of God and by the merits of Christ, withdrawn from the general law of original sin. Her redemption was the very masterpiece of Christ's redeeming wisdom.