A classic question is how could the Jews survive for so long? My answer is the Oral Torah and prophecy, but it is the exact opposite of the answers you’ll typically hear. Lets start with some history and how the Jews dealt with adversity.
I. The destruction of the first temple- For any other religion this would have been devastating the mainstream branch of the religion had the center of their religion despite prophets claiming it could never happen. However, there was a branch which had been prepared and either resigned themselves to religion in exile or accepted the prophecy that their grandchildren would see the next temple.
IIa. This is the first part of the second temple era- When the temple is destroyed you have a core of Jews who never came to Israel and who formed the core of the Talmudists and preserved the religion for another era. These Babylonian Jews are considered bad for not coming with Ezra, yet without them doing the wrong thing we very well may have no Judaism.
IIb. The codification of the Mishna- This was the adjustment made by Judaism after the destruction of the Second Temple. Rebbi Yehuda Hanasi could have easily held to his guns and tried to preserve an ever shrinking number of people who could remember all of the traditions and laws, but he didn’t and wrote the Mishna. I’m sure there were others who tried to live in the bubble, but they are long forgotten. (The same thing applies to the Talmud)
III. The separation of the European and the Middle Eastern Jews- If you look at the accomplishments of these two groups after the crusades you find that the European community contributed more to the fractal parts of the Talmud and had a more vibrant yeshiva enviroment, while the Middle Eastern Jews were far more involved in philosophy both rational and mystical. The European community from around 1100-1700 faced one challenge, in the form of the common Jew converting under pressure rather than dying. To deal with this philosophy was not of much value. What was of value, especially in a feudal culture, was a hierarchy of Rabbis who could bring the community together to make social outcasts of anybody who left the faith. To get this culture the hierarchy had to be one of knowledge and the Talmud is singularly fit for this purpose. On the other hand the Middle Eastern Jews faced the problem of assimilation of the elite who moved freely in the intellectual society. Therefore a Jewish approach to the rationality and the mysticism of the Arab world was developed. The other difference is the ubiquity of customs on the Ashkenazi side. These customs I believe were ways to maintain a community’s uniqueness and make the Christian world seem foreign and strange. Also, for instance, the law against polygamy may have been a way of keeping the problem of young men who leave the community to marry at bay.
IV. Enlightenment- This was a problem singular to Ashkenazi Jewry. One response was to assume that noting was wrong and to keep the basic structure of the shtetl including all of its problems. This approach suffered a heavy toll from its elite because of its high valuation of intellectualism. Those who saw the system and had previously stayed in because of intellectual arguments against Christianity now had an alternative which many went for (most of course did it out of apathy not careful thought). However in Eastern Europe the Hassidic communities weren’t hit as hard because they were heavily based on emotions. R’ Hirsch approach was to attempt to reconcile the Enlightment with traditional Judaism and it worked moderately well. The interesting thing is that the stay the course approach saw its failure and adjusted relatively quickly. Talmud was studied in a more systematic way as well as the mussar movement being quickly implemented. Without these changes it seems probable that the stayu the course branch would have withered away.
V. I’m skipping Zionism and going straight to the hot controversy, Modern Orthodox vs. Hareidism. If you look at the numbers you find that the Hareidi community is doing much better at achieving the main goal (keeping people frum). However their method, which is based on locking the outside world out will do this even if it is a failure. This means that although their community is not having the same attrition, they are more vulnerable to a devastating blow. I don’t know what would cause it. Possibly internet access which they aren’t able to block effectively. On the other hand it may be that Modern Orthodoxy won’t be able to withstand its attrition rate.
To summarize my opinion, Judaism has survuived because of all the spinoffs which either became the mainstream or forced the mainstream into a more healthy path. The other thing is that the main determinant of a spinoffs success is based on whether there is a significant problem in Judaism at the moment. Christianity survived and later thrived because the mainstream scene hadn’t dealt with the destruction of the Temple. Hassidus thrived because the Yeshivish world didn’t want to deal with the problems of the plebe. And Reform has thrived because there wasn’t a swift reaction to changing circumstances.