A person's religion is really just the sum of all of their beliefs. And peoples' beliefs are what drive all of the other things they say and do (including how they vote and participate in public life). So it's actually impossible to truly separate church from state in a lasting way.

So what we need to focus on instead, is spreading a culture (i.e. a religion) that respects democratically observable evidence, and rejects any claims that come from a prophet or "sacred" text.