The RAMBAM gives an amazing explanation of why HASHEM allowed Christianity and Islam to be so successful. Rabbi Miller OBM also had some very interesting ideas on this. We here offer an additional idea (all of them are true):
I think that HASHEM created Christianity and Islam to show us the contrast between Truth (Authentic Judaism) and SHEKER (falsehood, i.e. Christianity and Islam). This allows us to greater appreciate our TORAH and Authentic Jewish Ideology
We can see a small part of this contrast in how these contrasting entities dealt with false Messiahs. When JC was killed, his followers were shocked. They were anticipating a better outcome. But that did not materialize. So as the truism goes “When you cannot win, change the rules” and that’s exactly what the early Christians did. No more need for a Messiah who will be a Temple builder. No need for a Messiah who would gather together all the exiled Jewish people. No, none of that trivial stuff. Now, the Messiah’s main purpose was to die for other people’s sins and let them all off the hook. No more responsibility. No having to answer to a greater authority. Just do what you want (more or less) and believe in the dead Messiah and you will be ”saved”. Pretty cool deal, Uh?! (Isn’t it “interesting” that when JC was asked for a sign by the High Rabbinical Court, he replied that his “sign” would be that after being dead for three days he would get up. Ok, so the first place he should have gone after “rising from the dead” was to the SANHEDRIN (High Torah Court) and say: “Hey guys, here I am! But that didn’t happen, he showed himself to some women… you know the story…)
When Shabtai Tzvi was discovered to be a false Messiah, not one Authentic Torah authority “fiddled” with the definition of the real Messiah to cover their mistake (although there were indeed some fanatic followers of his who did this).
But when a certain individual named Ben Koziva (wrongly called Bar Kochba) was suspected to be some sort of Jewish Messiah and eventually was killed, no one fooled around with the rules. Nothing was changed. It was realized that he wasn’t the Messiah, and all Authentic Jews just kept on waiting for the Authentic Messiah.
The reason for this striking contrast is that Authentic Judaism is TRUE. Truth cannot be mutated. A universal truth such as Authentic Judaism doesn’t change its colors every few days. It NEVER changes.
If you are being threatened by Christianity, Islam or any other cult, contact these people immediately for guidance