That’s not how isms work and that’s definitely not what atheism means.
A+theosism. No. Atheos+ism. Yes.
It is the ism of being without god. The concept of god must exist in order for the person to think about and decide there isn’t one, ergo atheism. Theism if they decide there’s at least one.
There are other words that exist for what you’re trying to explain.
I had to really consider this, but I agree.
You can’t claim that a person is an atheist of they can’t conceive of a God. “Atheism” is ancient Greek for “without God.” A (without) and theos (God). The “ism” part was added much later.
You wouldn’t call ice cubes “without magma” because you wouldn’t be able to expect there to be magma.
You wouldn’t refer to an infant as “without Alzheimer’s” as it’s a ridiculous thing to clarify. So referring to ancient humans that predate the concept of religion as atheistic is a needless clarification and not a good argument for or against something, regardless.
Though I have to say the argument itself is super pedantic and probably not enriching anyone’s lives.
