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 this can be controversial, due to fact that in the time of founding fathers, the term "religion" was also a term for Christian denomination. In their time, a Baptist is in a different religion than a Methodist, because in their time, 99.8% of the U.S. was Christian)

Although nowadays associated primarily with Islamic states, 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 Church of England is an example. It is, however, also possible to have a national church which is not established, like the Church of Scotland, which has no constitutional links with the state. The Protestant Reformation was a movement which began in the 16th century as a series of attempts to reform the Roman Catholic Church, but ended in division and the establishment of new institutions, most importantly Lutheranism, Reformed churches, and Anabaptists. ... The Church of England is the officially established Christian church in England and acts as the mother and senior branch of the worldwide Anglican Communion as well as a founding member of the Porvoo Communion. ... The Church of Scotland (CofS sometimes known as the Kirk) is the national church of Scotland. ...

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. A chaplain is a priest or a member of the clergy serving a group of people who are not organized as a mission or church. ...

One controversy regards degrees of establishing religion vs. freedom of religion. Many religious denominations have similar beliefs, and have para-church bodies to represent their united interests. When beliefs common to multiple denominations are legally adopted, religious freedom will have decreased, but whether this is the establishment of religion is subject to debated interpretation. For example, belief in the Trinity could be made an oath of office, but this belief is part of many denominations, and does not require the state to support any single religious body. At the same time, it would significantly reduce the religious freedom of non-Trinitarians. Therefore the ability to control the official interpretation of the phrase "establishment of religion" creates political power and hostility. Note that this kind of denomination is not that of a coin or banknote. ... This article concerns the Holy Trinity of Christianity and related religious denominations. ...

See:

• First Amendment
• State religion
• Disestablishmentarianism
• Antidisestablishmentarianism