Religious fundamentalism is the type of religious behavior. It is a kind of religious movement or point of view characterized by a return to fundamental principles, by rigid adherence to those principles. Fundamentalism became a very extreme form of conservatism. It is often seen as violent, intolerant, stubbornly backwards.
The 'fundamentals' of a religion are held to be those morals, behaviours and beliefs held by the earliest followers, hence, fundamentalist ideas tend to clash with modern society and modern morality.