@cbontenbal@philosophy
Reading through the replies, I get the impression that you're asking what specifically atheists are not-believing in, is that right?
If so, I'd nominate 'personhood' as the defining characteristic of the god they reject. Rebuttals that point to 'uncaused cause' et al. don't seem to conflict with atheism in that sense, and often look like, 'uncaused cause -> ??? -> the Christian god'.
To say it differently: 'fire -> spark' rather than 'program -> programmer'.