1. an atheist,
2. an agnostic, or
3. a deist?
Well, the author's name is on Wikipedia's list of agnostics. Also, on its list of deists. Hmm. Now go looking for a list of famous atheists, and yup.
So what did Twain believe? Depends on whom you ask. In other words, it's complicated - but not to Colorado pastor Kevin Swanson. Swanson has gotten it into his head that Twain was one of the wickedest men who ever lived, on account of the fact that America's greatest humorist frequently made fun of religious phonies with observations like this one:
"In religion and politics, people's beliefs and convictions are in almost every case gotten at second-hand, and without examination, from authorities who have not themselves examined the questions at issue but have taken them at second-hand from other non-examiners, whose opinions about them were not worth a brass farthing."
Comment: Let's see, we have a global 'cultural' zombie craze, McNuggets and other fast food that is rumored to contain human flesh, and people eating other people...
McZombies, anyone?!