> it's trivial to prove there's no heaven or hell.

You're conflating physics and metaphysics.

> Flat earth is very similar to religion

You seem to have a beef to pick with religion. There are religious groups that do function similar to flat earthers, but that isn't true of all religious groups and many of the smartest and open minded people in history have been religious.

If anything, by making this comparison you are legitimizing idiocy.

There's a big difference between taking a position on unknowable metaphysical topics and refusing to recognize or even look at evidence when presented to you.

If you think I am legitimizing flat earthers you really don't get the point. I am just saying that for 90% of people belief in this or that mostly dictated by people around them.

> unknowable metaphysical topics

religious people often conveniently shift goalposts to make sure what they say is always beyond knowable. We launched satellites and found no heaven above? fine, it exists in some other way. No soul detected? our technology is not good enough. Flat earthers do almost the same thing just they use conspiracy theories instead.

If you want to find a difference between religion and flat earther theory, Christianity for example (not sure this applies to all religions) is supposedly helping humans live together better, like: be kind, do to others what you want be done to you, don't steal/kill/rape etc. But that's not really related to how it's proven or factual.