The most obvious answer is they're under the threat of hell if they dare to even doubt. But on consideration it goes much deeper than that. There's something in man's make-up that says 'threaten my beliefs and I'll hate you'. That seems to go for any belief or ideology-even political beliefs. You can see it even with a bumpersticker. Stick on "God is not a Republican' or that fish with legs that says 'Darwin' in the center, and watch the vicious scowls you'll get. You can see the hatred in their eyes.

That's why you have Jehods, witch hunts, snipers at abortion clinics.....the only sane people in the world are those who question EVERYTHING.