IMO, most real atheists reject spiritual hypotheses in favor of materialist alternatives, not philosophical "nothing"--and they must prove them against "nothing"--just like spiritualists.
I don't favor materialist alternatives. I am content with null hypothesis. Everything else is just believing whatever you think.
As close as I'll come to a belief system is to observe that consciousness seems like a lot of work and wasted time if it ends when we discorporate in this life. But it wouldn't surprise me, because it's not a belief.