IMHO, the metaphysical naturalist (atheist) claim to being rational rests on the worldview that: "all that there is" is all that exists in nature.
Well, "nature" really means that set of observed phenomena that have causes that are regular & predictable (at least in principle).
The atheist doesn't necessarily believe that there are no extra dimensions or different universes with different laws of nature or whatever operating. We believe there's no evidence of any "person" behind the existence of this universe. There has to be some evidence or good inferences that such things exist. IMNSHO :-) the belief that there's a person of some kind behind it all has no good evidence, and seems pretty obviously a case of anthropomorphism.
That worldview is not shared by many (if not most) physicists and mathematicians - the most epistemologically zealous of all the disciplines. It is also not shared by most philosophers and the general public.
OK, I'll have to bring in a physicist & some mathematicians on this. Guys, how would you characterize physicists & mathematicians in general WRT the existence of a supernatural world with a supernatural person governing our universe & lives?
betty boop and I are on the Plato side with Godel, Penrose, Tegmark and others.