On my brother-in-law-in-law's blog, some people got to wrangling over the LDS church's position on a proposed constitutional amendment to prohibit same-sex marriage. Some proponents of it played the religion card ("The prophet speaks, I obey," said one). My response:

See, policy we can debate. Even values we can debate. But once epistemology enters the picture, there's no way to be polite. "Your method of deciding what's true is wrong" or "I believe God is telling me the inerrant truth" has no place in civil discourse, yet religion's influence in politics means we have to deal with it all the time.

Once people have beliefs that we hold dear, that we specifically guard against change no matter what we learn or hear, how can we know they're true?

Critical thinking is our immune system against nonsense and fallacy. HIV kills the immune system - that's its dark genius. When your beliefs tell you that doubt is a moral danger, there is no way to have any kind of productive discussion, because you won't let yourself be changed.