A possible variant on the atheisk/athterisk, incorporating letters from the original Greek atheos.

Looks a lot more secret society, which might appeal to some, and also a nice reminder that atheism is not just a modern fad. Maybe it could even inspire some Christians to realize that were they living in the ancient world, atheism would have been the most correct choice, since polytheism is of course a load of balderdash. And then maybe the wheels would start turning in their heads, wondering if future generations might realize the same thing about today’s monotheistic religions– ha! Wishful thinking I know.