Faith cannot move mountains (though generations of children are solemnly told the contrary and believe it). But it is capable of driving people to such dangerous folly that faith seems to me to qualify as a kind of mental illness. It leads people to believe in whatever it is so strongly that in extreme cases thay are prepared to kill and die for it without the need for further justification ... Faith is powerful enough to immunize people agains all appeals to pity, to forgiveness, to decent human feelings. It even immunizes them against fear, if they honestly believe that a martyr's death will send them straight to heaven. What a weapon! Religious faith deserves a chapter to iteself in the annals of war technology, on an even footing with the longbow, the warhorse, the tank, and the hydrogen bomb.

Richard Dawkins, from 'The Selfish Gene'