If you deny or don't believe there is a supreme, higher, take your pick being then you're an atheist. If you don't believe, but also don't deny, you're not.
A person who holds that the existence of the ultimate cause, as god, and the essential nature of things are unknown and unknowable, or that human knowledge is limited to experience.
An atheist is a person who denies or disbelieves the existence of a supreme being or beings.
A person who just straight out doesn't believe.
No hypothetically speaking, no if's, no anything.