A funny conversation I was having with some friends today at lunch so I thought I’d bring it here.
We need something short and witty. Something we could put on a T-shirt.
I’ll get the ball rolling:
Atheist: Living guilt free since the “Big Bang”
I saw a pretty damn good one on a T-Shirt not so long ago… buggered if I can remember it though
Edit: Useful resource http://www.atheists-online.com/
How about “I don’t believe in God,” since that is the totality of atheist belief as such. (Obviously atheists may believe other things, like “All history is the history of class struggle” or “Hot dogs are tasty” but these beliefs are not applicable to all atheists.
“Of the 2500 deities in human history, I only believe in one less than you”
Or
“You’re only one god away from atheism”
Okay, probably not very catchy.
How about…
“I accept responsibility for my own fortune”
How about an agnostic one?
I don’t know and neither do you!
I thought not. 
“Atheist:For the same reason I don’t believe in Santa Claus.”
The Southpark episode about atheism had a nice one.
“Atheism: it’s not enough to not-believe; you also have to be a complete dick to everyone who disagrees with you.”
Never believe in Gods; it only encourages Them.
Oddly enough, most of the complete dicks I know are Christians.
If God Wanted People to Believe in Him, Then Why Did He Invent Logic 
Or:
Front: “There are 6000 religions in this world”
It’s too long to put on the shirt, but: "(with at least 100 followers and an dogma that differs from the others on at least one point).
Back: “At least 5999 of them must, logically, be wrong.”
Back, smaller print: “Pick VERY carefully.”
A man needs a god like a fish needs a net.
Meh . . . I’ll get back to you.
Intelligent Design: Fucking with your science class since 1987.
Can God make a fundie he can’t shut up?
There’s no God. It’s just the tooth fairy when he’s drunk.
Know God? No, no God.
SSG Schwartz
No atheists in foxholes. No Christians in whorehouses. Which would you rather be?
“The difference between God and Santa Claus is that Santa doesn’t tell you to kill people”
More years ago that I care to remember, we did an exercise in sloganeering as part of an advertising class. I like **Surby’s ** because it meets a lot of the criteria of the class, but I’d add one the element of curiosity:
“I don’t believe in Santa Claus, either.”
There are some other really good ones. **Linty Fresh’s ** deserves a close look; the mangled metaphor is classic sloganeering, and in this case **Linty’s ** dead on, I think. Deserves more than “meh.”
This is my favorite one so far. You could even condense it, and on the front of the t-shirt just have a list of the world’s religions in really small print.
Then on the back, ‘‘Pick VERY carefully.’’ I think it would get the point across nicely.
My favorite one, which might not be interpreted strictly as atheist, but really if you know anything about my favorite philosopher it’s pretty obvious, is
‘‘Nietzsche is my co-pilot.’’
Christianity: Because nothing screams “Mankind is evil” like a fucking piece of fruit.