Aberingi's long-running topic entitled "Do we need a Universal Atheist Symbol?" leads me to ask "Do we need a Universal Atheist Motto?".
Well, perhaps we don't, or perhaps we could, but some theists might get deservedly irritated if we chose to use something like "In Darwin we trust" at appropriate moments.
These words, suitably translated into other languages, have a basic international atheist appeal with respect to truth and helpful anti-theist sabre-rattling and debating.
e.g. they might look good on the side of a bus/ coach.