A person who believes in the basic, most important principles of a religion or ideology and follows them very strictly.
From 'fundamental' (basic principle) + '-ist' (one who follows). The term originated in American Protestantism around 1920 to describe those defending traditional Christian doctrines against modernism.
The word was invented for religious purposes but is now applied to anyone strict about their beliefs—from music fundamentalists to fitness fundamentalists. It shows how we borrow ideological terms when we want to describe intensity and rigidity in any area.
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