Fundamentalism is not religious extremism. It is not religion gone too far.
Fundamentalism is religion failing to happen. It is relationship to transcendent reality reduced to a set of defined things: facts, techniques, emblems, objects and social groups.
Of course, different denominations of Fundamentalism adhere to different things, but they all believe that religious “faith” consists of adherence to things, and they can’t see of what else a religion could possibly consist.
Religion begins when the limit of this vision is overcome, and then the limit of the consequent mystical vision is also overcome, and one plainly sees why love of God and love of neighbor are inseparable. And maybe it starts beyond that, too, and if that is true, neighbor, your vision penetrates further than mine, and we need to talk.