Pre-TLDR: I will be happy to accept if my hypothesis only holds in the weaker form of applying only to evangelical religions.

Yeah, I do think that that's a fair nuance to add to it and I was thinking whether I wanted to get into that in my original comment as well. However, and maybe it's just because I'm feeling particularly anti-theist lately, but I feel like the evangelical religions (And really there are at least two, aren't there? Muslims are generally pretty intent on forcing non-Muslims to live by their code) are just the more obvious presentation and maybe a more exaggerated presentation of the same principle.

The core problem is that any religion espouses an objective morality, an objectively correct way to behave. This means that as a religious person, although there may be some differences in behavior between me and others that don't fall under the purview of my doctrine, there will always be at least some behaviors that are mandated, and someone who behaves differently than me in those ways is objectively committing wrongdoings. I may be content to let them simply go on doing that, maybe. I believe their punishment will come to them in the afterlife, or something like that, but it seems to me like this would inevitably devolve into "I must force other people not to do X, even if they want to do X and it harms nobody, because it is evil to do X and they must be stopped". This is what I meant by saying that the most harmless form of religion is one that just hasn't followed its premises through to the rational conclusion.

Of course, it's a deep subject and there's a lot of other ways that it could evolve too. And it remains to be seen whether or not groups of humans would inevitably arrive at the same behavior under nihilism as well, just for different reasons. I.e " there is no objective wrongdoing, therefore if I don't like someone doing X, there's nothing wrong about me stopping them from doing it even if it harms nobody - just because I don't like it is good enough." You can maybe argue that one road being longer than the other makes it preferable... I'm really not sure. Definitely a lot of fun stuff to think about!