Andrew Sullivan has up a quote representing a defense of religion that's quite common:
Part of my skepticism with regard to the efforts of my fellow atheists to demonstrate how absurd the opposing position is comes from knowing a fair number of intelligent, reasonable, thoughtful people who believe in God--including one I am married to. Part comes from weaknesses I can perceive in the foundations for my own view of the world. At some point, I think, each of us is using the superb pattern recognition software that evolution has equipped us with to see a coherent pattern in the world around us--and since the problem is a harder one than the software was designed to deal with, it isn't that surprising that we sometimes get different answers.The idea seems to be that if otherwise great people believe something, it must be reasonable to believe. Once you've spelled it out like that, though, it starts to become easier to see the reasons why this is a dumb idea. People aren't representations of ideal types. They're people. They have good qualities and bad qualities. I've known religious people who've been great in their way. I've met similarly great people who believed in the effectiveness of tarot card reading, or who accepted Marxist dogmas about the nature of society without bothering to study economics or psychology or anything like that. Because they were great in other ways, it doesn't mean that it was good of them to accept those ideas. And Newton, though a great scientist and mathematician, was into alchemy and numerology. Who thought the world would end in 2060.
I'm not sure what else to say here. This is one of those issues where at first glance stating the obvious would seem to be enough. Of course, in reality stating the obvious is rarely enough. So let's speculate on why people think this way. I'm betting on the fact that as many have notee before, religious beliefs aren't even really supposed to be about reality. People don't rly on them the way they do their beliefs about reality, or when they do it's a tragedy. Rather, they're used for things like like establishing group solidarity. When you need to make a tightly knit group that readily hates outsiders, a unified dogma helps, but in a bourgeois society where you have to do business with people significantly different than yourself it helps to be able to adopt an irrational relativism about points of disagreement. To insist that your friends' belief in God is rational isn't to say it's really rational n the sense a philosopher would understand. It means you want a smooth relationship with them that ignores the question of whether anyone's beliefs about non-immediately relevant matters are true.