Mormons, members of the Church of Jesus Christ of Latter-Day Saints, practiced polygamy (having more than one spouse) in the early days of the church. In the Mormon church, husbands could take multiple wives. Congress, determined to outlaw the practice, passed the Morrill Anti-Bigamy Act of 1862 (not to be confused with the Morrill Land-Grant Act of the same year) targeting not only LDS marriage customs, but also attempting to prevent members of the Church and the institution, from accruing most of the land in Utah Territory. The practice of polygamy was anathema to certain mainstream Christian followers, so the laws may have had a significant basis in intolerance because they violated US social and religious norms.
Reynolds was the first test of the First Amendment Free Exercise Clause that guarantees the the freedom of religion.
“Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.”
On its face, the Constitution appears to forbid the US government from interfering in any aspect of religious practice. In Reynolds, however, the Waite Court held that Congress had no power to exercise control over opinions, but could limit actions that violated community social norms or subverted social order.
This decision created an exception to the First Amendment, demonstrating even constitutionally protected rights are not absolute.
Reynolds v. United States, 98 US 145 (1878)
For more information, see Related Questions, below.