Cummings v. The State of Missouri was a landmark Supreme Court case that established the principle of ex post facto laws. The case involved a Missouri law that prohibited anyone who had been convicted of a crime from practicing law. The plaintiff, William H. Cummings, had been convicted of a crime in 1858 and was subsequently denied a license to practice law in the state of Missouri. Cummings argued that the law violated the Constitution's prohibition against ex post facto laws, which are laws that retroactively criminalize actions that were not illegal at the time they were committed. The Supreme Court agreed with Cummings, ruling that the law was unconstitutional. The Court held that the law violated the Constitution's prohibition against ex post facto laws because it punished Cummings for a crime that was committed before the law was enacted. The ruling in Cummings v. The State of Missouri established the principle that ex post facto laws are unconstitutional and that individuals cannot be punished for actions that were not illegal at the time they were committed. This ruling has been cited in numerous subsequent Supreme Court cases and has become an important part of constitutional law.

In Cummings v. The State of Missouri, the Supreme Court was asked to decide whether a state law that prohibited clergy from teaching in public schools violated the Free Exercise Clause of the First Amendment. In an 8-1 decision, Justice Field delivered a dissenting opinion arguing that states have broad authority to regulate religious activities within their borders and that this particular statute did not violate any constitutional rights. He argued that while it may be true that some people are prevented from exercising their religion by such laws, they still retain all other freedoms guaranteed by the Constitution and can practice their faith freely outside of school settings. Furthermore, he noted that if every person were allowed to teach whatever religious beliefs they wanted in public schools then there would be no uniformity or consistency among students' education on matters related to religion which could lead to chaos and confusion in classrooms across America. Ultimately, Justice Field concluded his dissent with a reminder for those who disagree with such laws: “The remedy is not found in judicial interference but must come through legislative action”