Feast of Immaculate Conception

In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced anddefined 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."

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"If anyone does not wish to have Mary Immaculate for his Mother, he will not have Christ for his Brother."

"The Blessed Virgin Mary..."

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.

"...in the first instance of her conception..."

The term conception does not mean the active or generative conception by her parents. Her body was formed in the womb of the mother, and the father had the usual share in its formation. The question does not concern the immaculateness of the generative activity of her parents. Neither does it concern the passive conception absolutely and simply (conceptio seminis carnis, inchoata), which, according to the order of nature, precedes the infusion of the rational soul. The person is truly conceived when the soul is created and infused into the body. Mary was preserved exempt from all stain of original sin at the first moment of her animation, and sanctifying grace was given to her before sin could have taken effect in her soul.

source: Catholic Encyclopedia: Read More...