Populations tend not to be that static in religious affiliation. the first century AD's Jewish majority in Palerstine was a brittle thing, only recently enforced and still threatened by factionalism, reconversion, xenophobia and centrifugal tendencies. It's only straighforward from the outside, and at some point it is almost inevitable that the "real" Jews will start deciding who is a "real" jew and who isn't. The result is almost certainly a much reduced Jewish majority, even without outside forces intervening.
However, even with the diaspora in place you could still preserve that (though I suspect more plurality than majority), if you butterfly away the competition. The real problem is that for many centuries, Palestine was ruled by a succession of governments under which conversion away from Judaism carried considerable benefits. I know the remarkable heroic story of Jews clinging to their identity through centuries of persecution, but the fact remains that it is the story of a minority. We don't know how many Jews converted to become Christians or Muslims, but all evidence suggests that it was a lot. If remaining Jewish remains unstigmatised, or better yet, if it conveys privileges, it could stay the default religion of large areas of rural Palestine.
Here's an idea: the Jewish exemption from sacrifice. It only works if we posit a pagan Roman state either enforcing sacrifices earlier, or surviving longer. Jews were, by law, exempt from any form of religious worship to affirm loyalty to the emperor (in a lot of books it says "emperor-worship", but that's really too simplified. Emperor-worship was only ever required of very few people, and then by insane rulers). Now, Jewish-tinged or Jewish-derived forms of monotheism were popular in the Roman east. If the followers of these groups found they could integrate into urban and upper-class society and stay true to their faith by claiming that exemption, all these groups could stay under a big umbrella of Jewish identity. The Romans are very unlikely to care one way or the other. That could produce a form of Judaism that is diverse and rooted in folk tradition strong enough to reassert itself after multiple disasters.
Of course, this still requires future rulers to maintain that status, but it's certainly not impossible.