Fundamentalist (noun). An adherent of any religion who latches on to the extraneous details and decorative fripperies of the faith, to the utter neglect of the basic principles. A Christian fundamentalist insists on the literal truth of the creation hymn in Genesis while defying every precept in the Sermon on the Mount. An Islamic fundamentalist worries himself sick over whether the faithful can carry their groceries in paper bags (because infidels might have recycled the paper from old Korans, infidels being just the sort of people who would do that), while continuing to kick the poor and helpless in the teeth, which is the one thing known for a fact to make Allah mad as a hornet.
Etymologically, the word comes from a Latin term meaning having to do with the fundament.
Fundamentalists are unpleasant, but fortunately you will never meet one. The word is used in modern English only to describe the religious beliefs of third parties not present in the current conversation.