Among "developed nations", the US is one of the most religious. According to a 2002 study by the Pew Global Attitudes Project, the US was the only developed nation in the survey where a majority of citizens reported that religion played a "very important" role in their lives, an attitude similar to that found in its neighbors in Latin America.

Most U.S. citizens adhere to Christianity (78.5%). A 2001 survey found 15% of the population to have no religious affiliation, still significantly less than in other postindustrial countries such as Britain (44%) and Sweden (69%). Judaism is the second most prominent religion, with estimates ranging from 4.2 million (or 1.4% of the population) to 7 million (or 2.5% of the population). Other minority religions include Islam (about 2.4 million, or 0.6% to 0.7%, Buddhism (0.5% to 0.7%), and Hinduism (0.4%).

The U.S. religious marketplace is extremely volatile, with nearly half of American adults leaving the faith tradition of their upbringing to either switch allegiances or abandon religious affiliation altogether, a new survey found February 25, 2008.

Several of original 13 colonies were established by English settlers who wished to worship their own religion without discrimination: Pennsylvania was established by Quakers, Maryland by Roman Catholics and the Massachusetts Bay Colony by Puritans. The United States was one of the first countries in the world to enact a separation of church and state and freedom of religion. Modeling the provisions concerning religion within the Virginia Statute for Religious Freedom, the framers of the United States Constitution rejected any religious test for office, and the First Amendment specifically denied the central government any power to enact any law respecting either an establishment of religion, or prohibiting its free exercise. The framers were mainly influenced by Enlightenment ideals, but they also considered the pragmatic concerns of minority religious groups who did not want to be under the power or influence of a state religion that did not represent them.