In utopia, I think Agnoticism would be the default religion. No one would believe there was no god (as I do), and no one would believe there was a god. There would be no atheists and no theists, just agnostics
Personally, I'm a strong atheist because I believe, in my gut, that if god was rational, he'd show himself, and I like to think the rationality is good, and god is most often described as good. So, I believe that there is no god. But I also believe that the best belief system is agnostism, so In a very strange way, I'm atheist in order to convert people to agnostism. Kinda like some evangelists think that being a "good christian" is best, and are evangelival to the extreme in order to get people to "meet them halfway"...
In Discworld, I generally asume that most Atheists actually believe that "The gods are wrong" rather than "There are no gods". Also, belief in the Discworld, to me, at least, is a measure in faith in a god as something. For instance, in "Small Gods", Om is weak because although millions of people acknowledge his existence, only one actually has enough faith in him to think that he is the best god...
Oops, I've confused myself, I'd better shut up now
Sorry to offend anyone, but- Nevermind :-)