The First Amendment to the U.S. Constitution guarantees religious liberty with two clauses: “Congress shall make no law respecting an establishment of religion, nor prohibiting the free exercise thereof.” If asked what these two clauses mean, many Americans would respond that they mean whatever their authors meant when they wrote them. History makes plain what the founders meant. They intended for the newly-formed federal government to keep its nose out of religion within the various states, leaving the states to do just about anything they respectively wished.
Ten of the original 13 colonies had official (“established”) state religions, including Anglicanism, Congregationalism and Protestant Christianity in general. Under the First Amendment, the states remained free to designate official state churches. States could levy taxes in order to support the official state church, pay its ministers and build and maintain its meeting-places. So far as the First Amendment was concerned, states could discriminate against all forms of religion other than their own established churches. (Some states themselves passed laws that prohibited these things and they were free to do that as well.) But whatever individual states might do, the First Amendment said that the federal Congress — the law-making branch of the national government — could neither promote one form of religion above another (the Establishment Clause) nor hinder any particular religion (the Free Exercise Clause).
Today these constitutional clauses mean much more than that, for the Constitution is a “living” document — its words ever-applicable to new situations through the years. Because its authors did not know the future, they could not possibly speak specifically to every future circumstance. For this reason, the Constitution evolves with the changing times. Its ancient words take on new meanings for new situations. Often those new meanings would have been unimaginable to the founding fathers. That is not surprising, for time did not freeze at the beginning of the 19th century.
However, those new constitutional meanings and applications must build on legal interpretations from the past. This principle of following legal precedent is called “stare decisis.” Courts can modify former rulings. They can expand case law or limit it, affirm it or abolish it. When courts modify existing law, the existing law morphs into something new. Federal courts interpret the federal Constitution and the U.S. Supreme Court has the final word. In our system of jurisprudence, the Constitution legally means whatever a majority of the nine Supreme Court justices has most recently said that it means. There is no right of private interpretation here, not for regular citizens, not for lawyers, not for judges — not even for chief justices of state supreme courts. We might not like that or think that it is right. But whether we like it or not, that is the way it is.