Here in Nauvoo it was that the leaders of the church took the decisive step that carried Mormonism beyond the pale of the ordinary, tolerable, religious sects. Rumors of immorality circulated among the Gentile neighbors. It was bad enough, they thought, to have the Mormons chronic petty thieves, but the license that was believed to prevail among the leaders was more than could be endured by a community that did not count this form of iniquity among its own excesses. The Mormons were in general of the same stamp as their fellow frontiersmen until they took to this. At the time, all immorality was denounced and denied by the prophet and his friends, but in later years the church made public a revelation concerning celestial or plural marriage, with the admission that Joseph Smith had received it in the summer of 1843. Never does Mormon polygamy seem to have been as prevalent as its enemies have charged. But no church countenancing the practice could hope to be endured by an American community. The odium of practising it was increased by the hypocrisy which denied it. It was only a matter of time until the Mormons should resume their march.
The end of Mormon rule at Nauvoo was precipitated by the murder of Joseph Smith, and Hyrum his brother, by a mob at Carthage jail in the summer of 1844. Growing intolerance had provoked an attack upon the Saints similar to that in Missouri. Under promise of protection the Smiths had surrendered themselves. Their martyrdom at once disgraced the state in which it could be possible, and gave to Mormonism in a murdered prophet a mighty bond of union. The reins of government fell into hands not unworthy of them when Brigham Young succeeded Joseph Smith.
Not until December, 1847, did Brigham become in a formal way president of the church, but his authority was complete in fact after the death of Joseph. A hard-headed Missouri River steamboat captain knew him, and has left an estimate of him which must be close to truth. He was "a man of great ability. Apparently deficient in education and refinement, he was fair and honest in his dealings, and seemed extremely liberal in conversation upon religious subjects. He impressed La Barge," so Chittenden, the biographer of the latter relates, "as anything but a religious fanatic or even enthusiast; but he knew how to make use of the fanaticism of others and direct it to great ends." Shortly after the murder of Joseph it became clear that Nauvoo must be abandoned, and Brigham began to consider an exodus across the plains so familiar by hearsay to every one by 1845, to the Rocky Mountains beyond the limits of the United States. Persecution, for the persecuted can never see two sides, had soured the Mormons. The threatened eviction came in the autumn of 1845. In 1846 the last great trek began.