The Jewish exiles who returned to Israel, writes Stark, "were primarily those deeply committed to the Yahweh-Only Sect," and "they launched vigorous efforts to impose true monotheism on the entire society." And their vision of god had changed as well. No longer was Yahweh simply their national god, he was the god of all.
What happened next was that Judaism became centered on the Temple in Jerusalem. "The Judaism put in place by the Yahweh-Only Sect... required strict observance of the Law and absolute intolerance of polytheism. But authority over this new orthodoxy was centered in Jerusalem amd placed in the hands of a professional, hereditary priesthood." With the imposition of a mandatory tithe on the Jewish people, the Temple and its subsidized priesthood became "the dominant financial institution, acting as the state treasury as well as an investment bank."
Repeating an earlier observation, Stark argues that "pluralism is the natural state of any religious economy... It follows that religious monopolies can exist only to the extent that coercion is able to keep dissenting groups tiny and circumspect, and that whenever coercion falters, competing religious groups will arise. Because erstwhile monopoly religions inevitably are relatively lax, lazy, and worldly, most of their opposition will come from groups promoting a far more intense faith - from sects." While the present-day United States does not have nor ever did have a monopoly religious faith, Stark's theory seems validated by shifting religious demographics in the United States today, with the more liberal mainline Protestant denominations in decline while more hardline and conservative denominations like the Southern Baptists have increased in strength.
As for the Jews in post-Exile Israel, though the Temple priesthood was initially founded by those Jews from the Yahweh-Only Sect who had an intense commitment to their faith, "that religious intensity is never transmitted very efficiently from one generation to the next." Consequently, "Israel soon abounded in disputatious sects." Stark provides an overview of the three main Jewish religious groups that coalesced in the years leading up to the Roman era: the Sadducees, the Pharisees, and the Essenes.
In brief, the Sadducees were the Temple priests. The Pharisees were more numerous than the Sadducees. Stark writes that "[p]erhaps the most significant single contribution of the Pharisees was the establishment of synagogues in Israel." Synagogues were buildings used for local worship and the Pharisees held that a synagogue could be set up wherever there were at least ten Jewish men. Though the Sadducees were opposed to the practice of synagogues, the synagogues "became the primary institution of Jewish religious life" following the destruction of the Temple in Jerusalem in 70 C.E. The Essenes were an extremely ascetic group who are believed to have had a community at Qumran where the Dead Sea Scrolls were found.
From there, Stark expounds upon a little-known fact, that contrary to popular belief, that ancient Judaism was a proselytizing faith. This is contrary to the present day, where orthodox Jews tend to be rather insular. I once had an Orthodox Jewish co-worker who told me that his sect, the Lubavitchers, discouraged converts because of the strict requirements of the faith. However, Isaiah 49:6 has God telling the Israelites "I will give you as a light to the nations, that my salvation may reach to the end of the Earth." Given that the Earth is a sphere, one might be tempted to ask exactly where the end of the Earth is located.
During the Roman Era, it is estimated that anywhere from 10 to 15% of the population of the Empire was Jewish. Stark quotes an Adolf von Harnack's claim that "It is utterly impossible to explain the large total of Jews in the Diaspora by the mere fact of the fertility of Jewish families. We must assume... that a very large number of pagans... trooped over to Yahweh." From a practical standpoint, given the frequent conquests and occupations of the land of Israel since the time of the Assyrians, expanding a national religion by acquiring foreign converts could have the potential of providing a greater supply of manpower for an army of liberation.
But as Stark notes, there was in fact a two-way process going on. Just as Diasporan Jews were seeking converts among their Gentile neighbors, so were they too assimilating to the Gentile, particularly Hellenistic, culture of their neighbors. "All but a very few had so entirely lost their Hebrew that they worshipped in Greek and their scriptures had to be translated into Greek. Many Diasporan Jews, probably the majority of them, had abandoned some provisions of the Law. For example, the rules that made it very difficult to eat with non-Jews probably were widely ignored."The Diasporan Jews then were ripe for a reform movement in their religion that would jettison the strict requirements of Torah law while retaining their belief in what they believed was the one true god. And as luck or fate would have it, such a reform movement was just over the horizon.