A core point of existentialism is the need to act without certainty. Ie the dizziness of freedom, as Kierkegaard puts it. Fundamentalists are trying to satisfy

A core point of existentialism is the need to act without certainty. Ie the dizziness of freedom, as Kierkegaard puts it. Fundamentalists are trying to satisfy their need for certainty by pointing at a certain holy text and going "this is the Truth!". I think there has to be a way of satisfying the need for certainty without compromising epistemic rationality, however. I have yet to find it, though :(