Most Atheists define Atheism as “lack of belief in a God”, which seems like it applies here. Agnostic is usually defined as “lack of knowledge of a God”, which also works here, so both Agnostic and Atheist.
It makes sense when you look at Theist (with belief in a God) and Gnostic (with knowledge of a God) that adding an A before it just means “without”.
It was near the top of everything, or whatever we call /r/all over here, that’s where I saw it. I think it was in a technology community, but not 100% sure.