A fundamentalist is a person who adheres strictly to a set of religious tenets. The people who made up the word meant for it to sound simple and true: Fundamentals. Back to basics. None of this newfangled loosey-goosey stuff.
Unfortunately for them, it has also come to mean “rigid, unreasonable people who are convinced they’re right all the time.”
So an updated definition of “fundamentalist” might be: a person who adheres strictly to ANY set of beliefs, religious or otherwise.
But if you pay attention to the way people actually use it, you may find that what they mean by “fundamentalist” is often: a person whose beliefs are very different from mine.