Josephus also talks about Jesus-like Jewish martyrs that fought against Rome. Of course the cultural conditioning was in place, but that establishes only the plausibility of some minimalist version of the gospel story, not the actual historicity of that narrative.

I think Edward Gibbon perpetuated that view that Christianity is largely to blame for the collapse of the Roman Empire. But more recent historians point to lots of other reasons. The collapse of an empire is usually overdetermined, meaning that there are multiple causes that come together in a perfect storm.