fundamental in English means the core thing, the foundational thing, the most necessary and important thing.
When we add 'ist' to the word it deals with a way of thinking about something. This might be about Christian thinking -- so it would mean the most core, the most needful thing.
It could be about other ideas or topics also. For example: When it comes to education, I am a fundamentalist. This means I believe that the most needful things, the most basic things, the mains things are the most important. So then, if I am a fundamentalist about education, I will feel that it is not necessary to have a basketball team or photography club or some 'extra' things......the most important thing is just the basic education.
I hope this helps!
Alex