Establishment of religion refers to investing political power in a particular religious faith or body. Thus, an established religion is an officially favored religion, such as a religion that citizens of a nation are compelled to support, profess faith in, or follow.

Although nowadays associated primarily with Islamic states, or with Zionism in Israel, established religions have also been a hallmark of European countries, which have established various Christian churches, although the importance of official religions in Europe gradually dwindled after the Reformation. In some nations that still have an established church, the official status of the church is largely a technicality and freedom of religion is guaranteed. The United Kingdom is an example.

In the United States, the Constitution not only forbids the establishment of religion but also guarantees individual freedom of religion ("free exercise"). This clearly means the government cannot favor one faith over another. But some advocates believe it also should mean that the government must not promote religion in general. According to this view, the employment of chaplains in the military or by local police forces, even if all major denominations are represented, would be unconstitutional.

See:

• First Amendment
• State religion
• Disestablishmentarianism
• Antidisestablishmentarianism