Blood atonement was a doctrine in the history of Mormonism still adhered to by some fundamentalist splinter groups, under which the atonement of Jesus does not redeem an eternal sin. To atone for an eternal sin, the sinner should be killed in a way that allows his blood to be shed upon the ground as a sacrificial offering, so he does not become a son of perdition. The largest Mormon denomination, the Church of Jesus Christ of Latter-day Saints, has denied the validity of the doctrine since 1889 with early church leaders referring to it as a "fiction" in spite of evidence to the contrary and later church leaders referring to it as a "theoretical principle" that had never been implemented in the LDS Church.

Mormon fundamentalism is a belief in the validity of selected fundamental aspects of Mormonism as taught and practiced in the nineteenth century, particularly during the administrations of Joseph Smith, Brigham Young, and John Taylor, the first three presidents of the Church of Jesus Christ of Latter-day Saints. Mormon fundamentalists seek to uphold tenets and practices no longer held by mainstream Mormons. The principle most often associated with Mormon fundamentalism is plural marriage, a form of polygyny first taught in the Latter Day Saint movement by the movement's founder, Smith. A second and closely associated principle is that of the United Order, a form of egalitarian communalism. Mormon fundamentalists believe that these and other principles were wrongly abandoned or changed by the LDS Church in its efforts to become reconciled with mainstream American society. Today, the LDS Church excommunicates any of its members who practice plural marriage or who otherwise closely associate themselves with Mormon fundamentalist practices.