One who is narrow-mindedly devoted to their own ideas and groups, and intolerant of (people of) differing ideas, races, genders, religions, politics, etc.
One who believes dogmatically in something regardless of evidence or even conclusive proof that the thing is false or was staged; one who has true-believer syndrome.
Used other than figuratively or idiomatically: see true, believer.
A strict follower of a doctrine.