Reynolds v. United States was a Supreme Court case that addressed the issue of religious freedom. The case involved George Reynolds, a member of the Church of Jesus Christ of Latter-day Saints, who was convicted of bigamy in violation of a federal law. Reynolds argued that his conviction violated his First Amendment right to the free exercise of religion. The Supreme Court rejected Reynolds' argument, ruling that the federal law was constitutional and that the government had a legitimate interest in regulating marriage. The Court held that the First Amendment did not protect religious practices that violated the law. The Court also noted that the government had a legitimate interest in protecting the institution of marriage and that the law was not intended to target any particular religion. The Court's decision in Reynolds v. United States established the principle that the government could regulate religious practices that violated the law. The decision also established the principle that the government could regulate religious practices that were seen as contrary to public morality. The decision has been cited in numerous subsequent cases involving the free exercise of religion.

Justice Field delivered the dissenting opinion in Reynolds v. United States, arguing that religious belief should not be a factor when determining whether or not an act is criminal. He argued that Congress had no authority to pass laws regulating religious beliefs and practices, as this would violate the First Amendment of the Constitution which states “Congress shall make no law respecting an establishment of religion”. Justice Field further stated that while it was true polygamy was contrary to public policy, it did not necessarily follow that such behavior could be made illegal by Congress simply because it violated public morality or social order. As such, he concluded that any attempt by Congress to regulate religious beliefs and practices through legislation would constitute a violation of both the letter and spirit of the Constitution.