[I]n the darkest depths of their minds, almost all [fundamentalists] harbor deep, unspoken doubts about their belief system. In fact, for all their protestations about truth and certainty, doubt is perhaps the main wellspring of their zeal: only people who live in a perpetual state of guilty unbelief can be strung along for a lifetime in such a desperate, grasping quest for assurance that their faith is sufficient for a God who offers grace, but demands constant efforts at perfection in return.