In order to understand The Fundamentalist Church of Jesus Christ of Latter Day Saints one needs to take a look at the Mormon Church's history of doctrinal changes regarding the subject of polygamy. John Taylor, one of the Mormon Church's earliest leaders said:

We believe in honesty, morality, and purity; but when they enact tyrannical laws, forbidding us the free exercise of our religion, we cannot submit. God is greater than the United States, and when the Government conflicts with heaven, we will be ranged under banner of heaven and against the Government...

Polygamy is a divine institution. It has been handed down direct from God. The United States cannot abolish it. No nation on earth can prevent it, nor all the nations of the earth combined,...

I defy the United States; I will obey God.
Source: John Taylor (on January 4, 1880), President, Prophet, Seer, and Revelator, Church of Jesus Christ of Latter-Day Saints. Quoted in Under The Banner of Heaven John Krakauer, Doubleday (July 15, 2003)

"The Most Holy And Important Doctrine Ever Revealed"
The LDS Church happens to be exceedingly prickly about its short, uncommonly rich history - and no aspect of that history makes the church more defensive than "plural marriage."

The LDS leadership has worked hard to persuade both the modern church membership and the American public that polygamy was a quaint, long-abandoned idiosyncrasy practiced by a mere handful of nineteenth-century Mormons.

The religious literature handed out by the earnest young missionaries in Temple Square makes no mention of the fact that Joseph Smith - still the religion's focal personage - married at least thirty-three women, and probably as many as forty-eight. Nor does it mention that the youngest of these wives was just fourteen years old when Joseph explained to her that God had commanded that she marry him or face eternal damnation.

Polygamy was, in fact, one of the most sacred credos of Joseph's church - a tenet important enough to be canonized for the ages as Section 132 of The Doctrine and Convenants, on of Mormonism's primary scriptural texts.

The revered prophet described plural marriage as part of "the most holy and important doctrine ever revealed to man on earth" and taught that a man needed at least three wives to attain the "fullness of exaltation" in the afterlife. He warned that God had explicitly commanded that "all those who have this law revealed unto them must obey the same ... and if ye abide not that covenenant, then are ye damned; for no one can reject this covenant and be permitted to enter into my glory."

Source: Under The Banner of Heaven John Krakauer, Doubleday (July 15, 2003), pages 5, 6.

In 1856, recognizing the strength of the anti-polygamy vote, Republican candidate John C. Frémont ran for president on a platform that pledged to "prohibit in the territories those twin relics of barbarism - Polygamy and Slavery." Frémont lost the election, but a year later the man who did win, President James Buchanan, sent the U.S. Army to invade Utah, dismantle Brigham Young's theocracy, and eradicate polygamy.

The so-called Utah War, however, neither removed Brigham from power nor ended the doctrine of plural marriage, to the annoyance and bafflement of a whole series of American presidents. An escalating sequence of judicial and legislative challenges to polygamy ensued, culminating in the Edmunds-Tucker Act of 1887, which disincorporated the LDS Church and forfeited to the federal government all church property worth more than $50,000. With their feet held to the fire, the Saints ultimately had no choice but to renounce polygamy.

But even as the LDS leaders publicly claimed, in 1890, to have relinquished the practice, they quietly dispatched bands of Mormons to establish polygamous colonies in Mexico and Canada, and some of the highest-ranking LDS authorities secretly continued to take multiple wives and perform plural marriages well into the twentieth-century.

Although LDS leaders were initially loathe to abandon plural marriage, eventually they adopted a more pragmatic approach to American politics, emphatically rejected the practice, and actually began urging government agencies to prosecute polygamists. It was this single change in ecclesiastical policy, more than anything else, that transformed the LDS Church into its astonishingly successful present-day iteration. Having jettisoned polygamy, Mormons gradually ceased to be regarded as a crackpot sect. The LDS Church acquired the trappings of a conventional faith so successfull that it is now widely considered to be the quintessential American religion.

Having jettisoned polygamy, Mormons gradually ceased to be regarded as a crackpot sect. The LDS Church acquired the trappings of a conventional faith so successfull that it is now widely considered to be the quintessential American religion.
Source: Under The Banner of Heaven John Krakauer, Doubleday (July 15, 2003), pages 6, 7

Plural marriage was commonly practiced in Utah until the federal government made statehood contingent upon the abolition of polygamy. The church's president, Wilford Woodruff, issued a manifesto outlawing the practice in 1890, six years before Utah joined the union.

Mainstream Mormons hold that Woodruff issued the antipolygamy manifesto as the direct result of a revelation from God, not in an effort to curry favor with the federal government. "A key tenet of our faith is the belief in continuing revelation," says church spokesman Michael Purdy.
Source: Two Many Wives Valerie Richardson, Insight on the News. Volume: 17, Issue: 17. May 7, 2001.

Indeed, the doctrines of the Mormon Church can be - and have been - changed seemingly at will. LDS Church leaders often come up with 'new revelation' that directly contradicts earlier teachings allegedly revelead by the god of Mormonism. (In genuine Christianity, new revelation can not change or contradict God's written word, the Bible).

Mormon doctrines, teachings and/or revelations generally change - or are denied - when doing so is necessary or convenient (e.g. when it was discovered there are no people on the moon; when it became apparent that dark-skinned people do not turn fair-skinned by accepting the Book of Mormon; and when the US federal government forced the LDS church to abandon polygamy).

For instance, a full, well-documented account of the Mormon Church's changing doctrines and teachings regarding the subject of polygamy can be found in Chapter 9 of the online book, The Changing World of Mormonism This fascinating book documents the many changes and additions to various Mormon doctrines, teachings and revelations throughout its history.