I don't think "In Darwin we trust." is a good choice, as it gives primary attention on Darwin the man himself and secondary attention to evolution the the scientific theory itself, rather than the larger all-encompassing philosophy of skeptical inquiry and empirical evidence-based belief, which is much broader than Darwinian evolution.
My nomination: "By science we know."
Ha, ha! Not only Darwin, then it will be Einstein, Hawking, Dawkins, etc. we trust.
But no, we don't need any motto.
There are lots of good and thoughtful comments here, just as I would expect from this site, where I am entering my first comment myself having just joined.
I like the idea of invoking Darwin, but like others, don't want to have anything to do with emulating religions and the horrors wrought on mankind in their names. We also don't need any beatification of singular people, and I doubt that most of them would have wanted that either.
I'd like to throw a few classic Latin catch phrases out there:
"Dum vita est, spes est." - "While there is life, there is hope."
"Per aspera ad astra." - "Through difficulties to the stars." Or "For inspiration [look] to the stars."
I happen to like the second one for the wide range of meaning and almost universal connection to the atheist experience.
Hi everyone, I also feel that there is no need for a Universal atheist motto. My reason is that by putting forward a motto then atheism all of a sudden becomes an organized body of unbelief which is more or less analogous to organized religion, which I do not feel atheism is.
On the other hand, a majority of us atheists feel marginalized and downtrodden and we do need a way to speak out, encourage others or if we like, make fun of the theists. Like someone said earlier in the thread, if we were to start an ad campaign then it shouldn't be something that reeks of organization/organized atheism.