Fundamentalism usually has a religious connotation that indicates unwavering attachment to a set of irreducible beliefs. However, fundamentalism has come to be applied to a tendency among certain groups-mainly, although not exclusively, in religion-that is characterized by a markedly strict literalism as it is applied to certain specific scriptures, dogmas, or ideologies, and a strong sense of the importance of maintaining ingroup and outgroup distinctions, leading to an emphasis on purity and the desire to return to a previous ideal from which advocates believe members have strayed. Rejection of diversity of opinion as applied to these established "fundamentals" and their accepted interpretation within the group often results from this tendency.
Christian fundamentalism has been defined by George Marsden as the demand for a strict adherence to certain theological doctrines, in reaction against Modernist theology. The term was originally coined by its supporters to describe what they claimed were five specific classic theological beliefs of Christianity, and that developed into a Christian fundamentalist movement within the Protestant community of the United States in the early part of the 20th century. Fundamentalism as a movement arose in the United States, starting among conservative Presbyterian theologians at Princeton Theological Seminary in the late 19th century. It soon spread to conservatives among the Baptists and other denominations around 1910 to 1920. The movement's purpose was to reaffirm key theological tenets and defend them against the challenges of liberal theology and higher criticism.
By the late 1910s, theological conservatives rallying around the five fundamentals came to be known as "fundamentalists". They reject the existence of commonalities with theologically related religious traditions, such as the grouping of Christianity, Islam, and Judaism into one Abrahamic family of religions. In contrast, Evangelical groups (such as the Billy Graham Evangelistic Association), while they typically agree on the theology "fundamentals" as expressed in The Fundamentals, are often willing to participate in events with religious groups who do not hold to the essential doctrines.
The Shia and Sunni religious conflicts since the 7th century created an opening for radical ideologues, such as Ali Shariati (1933–77), to merge social revolution with Islamic fundamentalism, as exemplified by the Iranian Revolution in 1979. Islamic fundamentalism has appeared in many countries; the Wahhabi version is promoted worldwide and financed by Saudi Arabia, Qatar and Pakistan.
The question of the relationship between Germanic neopaganism and the neo-Nazi movement is controversial among German neopagans, with opinions ranging across a wide spectrum. Active conflation of neo-fascist or far right ideology with paganism is present in the Artgemeinschaft and Deutsche Heidnische Front. In Flanders, Werkgroep Traditie combines Germanic neopaganism with the ideology of the Nouvelle Droite.
In The New Inquisition, Robert Anton Wilson lampoons the members of skeptical organizations such as the Committee for the Scientific Investigation of Claims of the Paranormal as fundamentalist materialists, alleging that they dogmatically dismiss any evidence that conflicts with materialism as hallucination or fraud.
Political usage of the term "fundamentalism" has also been criticized. "Fundamentalism" has been used by political groups to attack their opponents, using the term flexibly depending on their political interests. According to Judith Nagata, a professor of Asia Research Institute in the National University of Singapore, "The Afghan mujahiddin, locked in combat with the Soviet enemy in the 1980s, could be praised as 'freedom fighters' by their American backers at the time, while the present Taliban, viewed, among other things, as protectors of American enemy Osama bin Laden, are unequivocally 'fundamentalist'.
The Associated Press' AP Stylebook recommends that the term fundamentalist not be used for any group that does not apply the term to itself. Many scholars have adopted a similar position. Other scholars, however, use the term in the broader descriptive sense to refer to various groups in various religious traditions including those groups that would object to being classified as fundamentalists, such as in The Fundamentalism Project.