The essential nature of fundamentalism is NOT a literal belief in this or that holy text. Fundamentalism, at its root, is the belief that there is only one way to think about and understand reality. And the fundamentalist, of course, believes he or she possess that one correct understanding.
This kind of thinking is not merely egotistical and childish, it is dysfunctional and disabling. It disallows the person any capacity to achieve honesty, accuracy, and critical reflection in their worldview. As a result, wherever they step outside their cohort, this kind of disabled thinking can poison their human relations and incline them into actions of emotional, moral, and physical violence. And in this way, it does great damage not only to the individual fundamentalist, but to all parts of the world they touch.
Such thinking is not limited to religious people. Most ideologues are, at heart, fundamentalists, and, ironically, I know many fundamentalist atheists. Indeed, the political landscape of the world in in this era is profoundly poisoned by fundamentalists of many stripes, as the center is being abandoned to serve anger and intolerance.