Early Judaism revolved around sacrifices at the Temple in Jerusalem. Donations from Judea and from from Jews abroad were used to purchase sacrifices at the Temple according to scripture. While there were some synagogues they were mostly used for reading the Torah and not used for prayer. When the Emperor Titus destroyed the second Temple in 70 CE Judaism and its followers had a problem since there was no Temple. The Jewish religion underwent a reform at this time. Synagogues were now used for prayer and the emphasis went from sacrifice at a single site to a distribution of religious activity to all areas. This is the beginning of the Rabbinic period and new teachers called Rabbis appeared. There is no mention of Rabbis in the Bible but the Jewish religion had to adapt to conditions. A public prayer group only needs 10 people (at first they had to be men) and they can meet anywhere. No Rabbi must be present and no sermon must be performed.
(The Great Courses – Judaism)