Fundamentalism, in many ways, used to rely solely on the inherent truth of its system as the source of its certainty.
It has become more complicated. Now, fundamentalism, although it still depends on the inherent truth of its system, is more of a reaction to the increasing mystery and uncertainty of our world. Fundamentalism is only going to become more rigid as our world becomes more unpredictable. As innumerable questions mount, the ground of fundamentalism will rise to meet them.
I have seen this in my own life as well as in others: I’ve seen conversion to fundamentalism when the questions and uncertainties are just too overwhelming to be assimilated by the mind. I’ve also seen conversion away from fundamentalism for the exact same reasons.
Which makes one wonder if the actual root of fundamentalism is fear.