Ineffabilis Deus

Ineffabilis Deus (Latin for "Ineffable God") is 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:

"Mary's privilege ... was the result of God's grace and not of any intrinsic merit on her part", which is reflected in the decree. The 1964 Dogmatic Constiution, Lumen Gentium noted the view prevalent "...among the Fathers whereby they called the mother of God entirely holy and free from all stain of sin, as though fashioned by the Holy Spirit and formed as a new creature."