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Ineffabilis Deus (Latin for "Ineffable God") is the name of an Apostolic constitution by Pope Pius IX. It defines the dogma of the Immaculate Conception of the Blessed Virgin Mary. The decree was promulgated on December 8, 1854, the date of the annual Feast of the Immaculate Conception. Mary's immaculate conception is one of only two pronouncements that were made ex cathedra and is therefore considered by the Catholic Church to be infallible through the extraordinary magisterium.
The decree surveys the history of the belief in Christian tradition, citing its roots in the long-standing feast of the Conception of Mary as a date of significance in the Eastern and Western churches. It also cites the approval of Catholic bishops worldwide who were asked in 1849 to offer their opinion on the matter.
The dogmatic statement is expressed near the end of the document:
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 entire decree is also worded to suggest that Mary's Immaculate Conception was not in any sense a logical necessity, but rather a divine gift to Mary as Mother of God.
- Immaculate Conception
- List of papal bulls
- Marian papal encyclicals and Apostolic Letters
- Papal infallibility
- "The Immaculate Conception." Papal Encyclicals Online. Retrieved December 7, 2012
- Digitalnun. "The Immaculate Conception of the B.V.M." iBenedictines. December 8, 2011. Retrieved December 7, 2012.
- "Are papal encyclicals infallible?"  Our Lady of the Rosary Parish Bulletin. January 1996. Retrieved December 7, 2012.
- Richert, Scott R. "Immaculate Conception" About.com: Catholicism. Retrieved December 7, 2012.
- The Decree 
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