“adults either know or don’t know” then you immediately acknowledge there is room for ambiguity. Revealing that you worked backwards from the premise of “belief in God is childish”.

Why else would your example of a reasonable “adult” usage of believe be valid but not

“Hey man is there a god” “I believe so but I haven’t seen him personally”

In both examples there is a being/place beyond our current reach which we believe to be there, for whatever reasons, but are unable to confirm at the moment.

Ironically in your rush to call others childish you posted the most childish response here by assuming your understanding is the only valid one.