fundamentalist
n.someone who follows the basic rules of their religion or beliefs very strictly. They usually believe that their holy books are completely true and should not be changed.
n.an adherent of a movement or point of view characterized by a return to fundamental principles and rigid adherence to basic tenets. Often carries a pejorative connotation when used by outsiders to describe religious or political extremism.
“The fundamentalist refused to accept any changes to the ancient laws.”
“Sociologists studied how the fundamentalist group maintained its strict social boundaries in a rapidly modernizing city.”
Often functions as a modifier before another noun (e.g., 'fundamentalist views').
“He argues that by their nature, fundamentalists (Christian or Islamist) are particularly vulnerable to exposure, discrediting, and blackmail because of their rigid stated views on sex.”
From fundamental + -ist, after a book series called “The Fundamentals: A Testimony to the Truth“ (1910).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.