Shortly after the adoption of the U.S. Constitution in 1788, the new nation ratified a Bill of Rights whose first order of business was freedom of religion. The First Amendment laid down what was then a bold precept: the United States would have no established, or officially endorsed, religion, and it would permit the free exercise of religion. More than two centuries later more religions are being freely exercised than the nation's founders could possibly have anticipated. Every substantial religion in the world has an American manifestation, and many homegrown startups have appeared in the United States. It is safe to say that no place in the world has greater religious diversity than the United States at the dawn of the twenty-first century.

Terminology

One small but vital part of that diversity consists of what are...