Explanation: In pre-modern polytheist faiths in general, but Hellenic religion in particular, there is a tendency to adopt the gods of one's neighbors - or rationalize them as the same god under different names. To the Romans this was the Interpretatio Graeca/Interpretatio Romana, depending on what time and cultures you're discussing. To the Romans, the gods were the same all around the world, just worshipped and seen differently. This made it very easy for Romans to spread their own practices and to adopt the practices of other faiths - Roman gods usually had many epithets representing their varying aspects (MARS SILVANVS for Mars as god of the woods, for example), so to tack on a provincial name was no problem at all - MARS LENVS was both Lenus, Gallic god of war, and Mars, Roman god of war; the Romans saw no contradiction in it.
Furthermore, to the Hellenic faith of the tradition practiced by the Greeks and Romans, correct belief ('orthodoxy') was of secondary importance - correct ACTION ('orthopraxy') was what was vital. Usually envisioned as 'paying the proper respects to the gods' rather than a united moral code.
On one hand, this is a very tolerant outlook, and that is commendable. On the other hand, it could lead to some curious ideas about other faiths - ranging from the eccentric (equating Wodan to Mercury because they're both traveling gods) to the outright offensive (asserting that the God of the Jews, YHWH, was just IVPITER CAELVS, Jupiter as envisioned as the sky itself, since the Jews didn't and don't make idols of their God).
Indeed, most religions are quite tolerant of other religions.
Christianity and Islam are the two biggest exceptions. They are religions of empire and designed to spread along with empire.
This is not to say that people of other religions can't be violent or intolerant, but as an example, you can find quite a lot of Hindus who also have a picture of Jesus or Mary in their shrine.
I firmly believe that the original Christianity was not like this. It became like this after the Romans leveraged it as a state religion 300 years after it started. And Mohammed copied that Roman idea another 400 years later.
Well, at least part of that aspect of Christianity is preserved from early Christianity. The Romans didn't like Christianity because it hit almost all of their taboos at once - and there are entire books written on that, so I won't get into the other 'triggers', if you will - but one of the BIG taboos it hit was refusing to pay respects to the gods. For the Romans, the idea that the Christians 'didn't believe' in the Roman gods was irrelevant - hell, the great Roman senator and orator Cicero was an atheist and a priest - but that they refused to swear oaths by Roman gods and refused to participate in pagan rituals under any circumstances was considered downright dangerous.
They might have escaped, even with all their other problems (secretive meetings, closed community, foreign, one God, popular amongst slaves) because of their associations with the Jews, whose religion the Romans tried to respect (even if they didn't always understand it well), and who were exempt from certain requirements that other subjects of the Empire were subject to. However, the Jews and early Christians were very much opposed to one another, so Jewish communities across the Empire not only refused to speak up for early Christians, but often were the ones bringing complaints in the first place.
Always! I love sharing what bits and pieces I've picked up over the years!
I'm honestly not sure what the relation of early Christian theology is wrt hell as we would understand it in the modern day. I know by the 4th century it was a 'thing', but other than that, I'm afraid I don't have an answer.
There are many aspects of Christianity today that would be unrecognizable to early Christians, but belief in hell probably isn't.
There are the usual caveats about when passages of the Bible were actually written - the canonical new testament wasn't solidified until long after when Jesus was supposed to have lived, and it's understood even among Christian scholars that books attributed to one author (like the gospels) actually draw from multiple earlier texts.
All that said, in Luke Jesus tells a parable about a rich man in hell asking a poor man in heaven to go and warn his friends so they don't also end up in hell.