After Talmudic times, until the late seventeenth  century, communities (and rabbis) were fairly independent. True, there were times and places where a group of cities, or even an entire country, would get together in a confederation, complete with  a central rabbinic/legislative council, and establish common practices, but these were not the norm. Occasionally, a great figure would have influence far beyond his own country, but it was up to the local rabbis to give their imprimatur to those decisions. Community councils chose the rabbis, taking their views and writings into account when deciding whether to accept him as rabbi. The standard practice in most places was that a young scholar would study for a number of years under the rabbis of his region. Upon completion of his studies (formalized in Germany in the 14th century with an official ordination),  he would ask an accepted rabbi of a community to accept him as an intern; seeing and partaking in every day rabbbinic duties and decision making. When that rabbi felt him to be ready, he would grant the young man an additional ordination. The young rabbi would then seek an appropriate community where he could serve. There was no such thing as the rabbi of a synagogue. That only came in fairly recently. The rabbi was responsible for governing the community (in tandem with the elected city council), making decisions in halachah that were binding upon everyone, deal with people's individual issues, perform marriages, divorces, and all other religious ceremonies; as well as oversee the synagogues, local schools, slaughter houses, butcher shops, and mikvehs. There were no formal Yeshivot (Rabbinic Academies) at that time, although many students would often gather to a particularly great luminary, forming a sort of ad hoc Yeshivah around him.
Everything changed in the late 1600s. Although we have had many false Messiahs in our history, there was only one who garnered near universal acceptance, sending world Jewry into a tailspin upon his conversion to Islam in 1666. His name was Shabbetai Tzvi. I hope to reprint some essays about him which I wrote some months ago. Like a great hurricane that reshapes the map of an area, the Jewish world was never the same. There was no country spared. Famous rabbis had also erred. There was a crisis of faith. Could even rabbis be trusted? (There are still some nine thousand followers of Shabbetai Tzvi in Turkey).
Among the after-effects of this debacle, a number of things changed. Jewish authority shifted away from the local city or town, in the direction of centralized Batei Din (Courts). These batei din were made up of the brightest and best scholars in the region, with the highest standards of integrity. To be sure, there were local rabbis. But more and more they deferred to the decisions and policies of the court of "Gedolim" (Great ones). Even "world class rabbbis" who were not members of a Beit Din wielded great sxwsay over lesser rabbis This became known as the system of Daat Torah (Daas Torah in Ashkenazi pronunciation). Formal institutions grew around these central authorities, insuring that their students would be well trained in sources, and not fall sway to popular delusions, like the Shabbaetai Tzvi tragedy.The great rabbis have hearts and minds filled with Torah. Shouldn't they be the people we turn to in cases of doubt? Beyond that, it was felt that these men should be the ones to ask even on questions that were not directly religious ones. Should I go into partnership with X? Should I marry Y? These were questions that had not classically been the province of rabbis. But there could be little doubt that advice given by a true Torah scholar, with no personal connection to the situation, would likely to be far better than that given by random friends.
This system has become well rooted in most Ashkenazi Orthodox communities until this day. I have had the experience of being asked by congregations where I had applied for the position of rabbi, "when we ask you a question, who will be answering it?" In other words, which Gedolim do you follow? But this system also has its critics. Why? That will be my next post.