That’s effectively trying to prove a negative, which is nigh impossible.

Also, it’s commonplace for a believer te be unable to empathize with a nonbeliever: it’s unfathomable to them that non-belief could be possible.

It’s akin to a person with loving parents to ever consider that their parents could be unloving, their minds can’t comprehend it.

So the believer goes about life thinking everything is a belief, even atheism. Which is an incorrect assessment.