Since the Founding era, neither the state nor federal governments have enacted many laws that intentionally discriminate against particular religions, at least not openly. Lawmakers know that such laws are clearly unconstitutional. So how do Free Exercise cases arise? They typically occur in situations like the polygamy case discussed in Reynolds v. United States. That is, followers of a particular religion will demand a religious accommodation—an exemption from a generally applicable law, like the prohibition of bigamy—that they claim infringes their ability to freely practice their religion.
Over most of the 20th century, the Supreme Court followed the general approach it had used in Reynolds v. United States: it balanced the Free Exercise rights of the religious practitioners against the interests of the state. If the state’s interest was strong enough, no religious accommodation was granted. But this balancing approach led to unpredictable and sometimes inconsistent results.
One of the more important Free Exercise cases of the 20th century was Sherbert v. Verner (1963), in which the Court decided that a Seventh-Day Adventist who could not get hired because she refused to work on Saturdays could nonetheless collect state unemployment benefits. In Sherbert, the Court made it clear that, in order to deny a religious accommodation, the government had to pass the “strict scrutiny” test. That is, the government had to demonstrate that denying the request was 1) the least restrictive means, and 2) had a compelling government purpose.