Gibbon also made the interesting point that Jews reject all other cultures and peoples in a way that was somewhat unique in the ancient world. I tend to think of them being rejected, as in anti-Semitism, but they equally reject everyone else as Goyim. I have never really understood anti-Semitism, but it is a fact of human psychology that people do not like people who look down on them, which can be inferred from the fact that no Gentile could be good enough to marry a Jewish girl.
He also noted as one of the reasons for the triumph of Christianity, which first occupied the entirety of the old Roman world, and has since of course spread over the world, was its intolerance. Competing creeds were banned and quite often punished violently, which had not been the case with Roman religions, which were very pluralistic and syncretic. We think of Christianity as a creed of love, but its actual history is one of more or less direct imperialism, both politically and culturally.