Hell seems like the kind of tool that a conquering army would use to intimidate people.
Tbh a belief I've had for quite a while is that many religious norms and tenets hold not really a "spiritual" use, but rather a very pragmatic one. Nothing about God giving us orders but rather something that had a practical application here on Earth, certainly at the time the religion was "invented".
My go-to example is the Jewish and Muslim restrictions on various food, born not out of God giving us orders but rather because poorly cooked pork or whatever could have given you an illness back in the day. Of course it isn't applicable now but the religion has stuck with that.
Hell indeed is just another tool like that. It's much easier to convert people to your religion when you tell them that if they don't, they'll burn in hell for eternity rather than telling them "sure, it's fine if you don't convert
".
Of course the
real incentive wasn't hell or the lack thereof but rather the invading army which has been destroing villages in your kindgom or whatever. I find "Convert or we'll burn you at the stake" more convincing than "convert or you'll go to hell when you die" personally
