Some of the classic works of Judaism (Strack and Billerbeck or their predecessor Lightfoot) are based on the idea that Mishnah and Talmud can be mined for “background” for the teaching of Jesus or Paul. Gabriele Boccaccini has this older scholarship in mind when he begins his book on the Roots of Rabbinic Judaism with the observation that “the idea that already during the Second Temple period Rabbinic Judaism was normative or mainstream belongs to the history of scholarly research.” This is a point with which virtually all of scholarship would agree, yet the average pastor still uses tools that have not quite caught up to scholarship. It is easier to read Edersheim’s Life and Times of Jesus the Messiah than to wade through E. P. Sanders’ Paul and Palestinian Judaism!
One of the biggest mistakes Christians make when they read the New Testament is to imagine that the Jews of the first century had a single set of unified beliefs and practices, something like a Creedal Statement or a “doctrinal statement” found in the later Christian church. This is simply not the case, although there were certainly a few doctrines which made the Jews unique and practices which set them apart from the Gentile world. For the most part, Judaism at the beginning of the first century was a fractious as Christianity was by the end of the century.
A second mistake most Christians make is thinking that modern Judaism is a good representation of how Jews lived in the first century. Unfortunately Fiddler on the Roof may not be a good guide to how Jews lived in first century Galilee. The Mishnah was not developed until A.D. 250 and the Talmud several hundred years later. Many of the things Christians say about “all Jews” are drawn from these documents but may not reflect the practice of the first century at all.
Having made these two caveats, the main theological doctrines which “all Jews believed” in the first century were basically monotheism and the election of Israel. Each of the “parties” within first century Judaism believed that there is only one God, and he is the God of Israel. Each of these parties equally affirmed that God has chosen Abraham and his family to be the recipients of his Law. While the relationship of God and his people will vary, that God has specially chosen Israel is foundational.
The main practices which defined Judaism included circumcision, food laws, purity laws, and Sabbath. With respect to theology, you either believe the Lord God is one, or you do not. You either believe God has chosen Israel to be his people or not. But with respect to practice, various groups struggled to find a balance between living as a Jew and living in a Greco-Roman world. How much effort was put into keeping the Law varied depending on one’s commitment or even one’s proximity to the Land. A Jew living in Ephesus may not have considered the purity laws as important as someone in Galilee, or Judea, or in Jerusalem itself. The closer to the Land, the closer to the Temple, the more likely ceremonial purity was an issue. Keeping Sabbath in Ephesus was much more difficult than in Jerusalem.
It is therefore little wonder that when we read Paul in Galatians, for example, food and circumcision are the two biggest issues between Paul’s Gentile mission and the so-called Judaizers.