Wikipedia introduces the subject thusly:
Fundamentalism usually has a religious connotation that indicates unwavering attachment to a set of irreducible beliefs. However, fundamentalism has come to be applied to a tendency among certain groups—mainly, though not exclusively, in religion—that is characterised by a markedly strict literalism as it is applied to certain specific scriptures, dogmas, or ideologies, and a strong sense of the importance of maintaining ingroup and outgroup distinctions, leading to an emphasis on purity and the desire to return to a previous ideal from which advocates believe members have strayed. Rejection of diversity of opinion as applied to these established “fundamentals” and their accepted interpretation within the group is often the result of this tendency.
I think the above probably states how most people understand fundamentalism, so rather than play with the definition, I will try to explain why it exists, and why is seems to be the source of so much disquiet in the world.
The first people who called themselves ‘fundamentalist’ were Christians in the late 19th and early 20th centuries, who were reacting to the liberal movements in theology of the late 1800s; so it is a rather recent development. Later this term was applied to other religions and, eventually, to certain non-religious groups that were reacting against the threat of change from valued norms.
The term fundamentalism has come to have negative connotations for us today, due to the actions of extreme minorities within some of these groups, most recently Muslim fundamentalists, but certainly not restricted to Islam. The common thread that links them is fear combined with faithlessness.
Fear of change is natural, especially fear of change to pillars of people’s perception of the world and their place in it. This kind of reaction was evident after Darwin published his theories about evolution. Some saw in this new view of the world a threat to their biblical version of how life came to be at told in Genesis: ‘if the Bible is in error in Genesis, then how could one trust anything in the Bible?’ Although, the reactionaries were not called fundamentalists, they fit the definition.
When one’s essential, basic beliefs are challenged, it is normal to fear. It is as though the floor is taken away and there is nothing but empty space below. For most people, faith exists as an antidote to fear, but when the fear is about the basis for one’s faith, whence comes the faith needed to cope? The Fundamentalists thus clung tenaciously to that which they believed was the foundation of their faith, although really this was an illustration of their lack of faith outside their narrow set of beliefs.
Real faith is not based on specific beliefs, and in fact, needs no object at all. Real faith is not ‘belief in’, but simply belief. When one does not have access to a faith without an object, one is faithless.
Cast into the situation of fearfulness without faith to moderate it, one’s reaction is to fight. Unfortunately, the fight often spills out into church politics, the wider society, and occasionally, onto the world scene.
It is no news to anyone that extreme fundamentalists can be dangerous to others, but they have become so because they function on the basis of a cold ideology and one that is exceedingly narrow as well. Without the capacity to see the nuances in every situation, they cannot account for variations from one individual situation to another and so cannot allow for compassion to intrude on their ideologically-scripted judgments. Fundamentalists also miss out on personal/spiritual growth for themselves, for in order to grow, one must be able to put present beliefs aside in order to incorporate new truths.