I think the Zoroastrian belief in an afterlife of eternal torture for the sinful predates the Christian one by several centuries. And the ancient Greeks – who didn't exactly have a barrel of laughs lined up in their afterlife for anybody – stipulated eternal punishment for some notable individuals like Tantalus and Sisyphus.
The problem with talking about any religion is that phrases like "the Christians" don't actually mean anything. There are no objective measures of religion: you can't give a blood-test to check for Christianity, for example. They are subjective definitions only: even within dogmatic organisations like the Catholic Church, they're constantly in flux – if only because the dogmatic ones concoct all manner of rules to try to cover every eventuality, which just guarantees eventual failure. You wouldn't have to go too far back from the current Pope to find one of his predecessors who would condemn him for his heretical beliefs on some point of doctrine or other.
You can't even appeal to certain basic principals, such as (keeping to Christianity) belief in the crucifixion and resurrection of Jesus, as being definitive. It's entirely possible for someone to regard the whole New Testament as purely symbolic, as simply a collection of myths with no basis in fact, and yet consider themselves, by their own measurement, to be a Christian. It's not like it's a copyrightable concept, despite what some (again, self-defined) Christians might want to think ...
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