Zzyzx wrote:What reasons can be identified to account for the decline?
My favorite theory is that the Republicans have tainted the brand. They claim to carry the banner for Christianity (like you can't be a real Christian if you don't vote for their candidates) and then they behave so reprehensibly that people are repelled. People wind up thinking, "If that's Christianity, I don't see the appeal."
And of course, since Christianity has no logical appeal, once you destroy the emotional appeal, there's nothing left.
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A note on the impact of the internet. I predicted that it would cause a "Cambrian explosion of microcultures"; it would let people find like-minded people to interact with, regardless of how weird their beliefs.
Only, in the case of atheism, the belief turned out to be not-that-weird. When I was a kid in Kansas, I didn't know of a single other atheist. I'd read about Madalyn Murray, but she was the only other one I'd even heard of. (Not counting the Russians, of course.)
The internet let the cat out of the bag: We are everywhere. We are normal. Christianity is no longer a default, something to be chosen automatically.
The internet played a considerable part in this change.