Everyone knows that Judaism gave birth to Christianity. But the formative centuries of Christianity also tell us much about the development of Judaism. As formative Christianity demands to be studied in the setting of formative Judaism, so formative Judaism must be studied in the context of formative Christianity.
Both Judaism and Christianity rightly claim to be the heirs and products of the Hebrew Scriptures—Tanakha to the Jews, Old Testament to the Christians. Yet both great religious tradition derive not solely or directly from the authority and teachings of those Scriptures. They reach us, rather, from the ways in which that scriptural authority has been mediated, and those biblical teachings interpreted, through other holy books. The New Testament is the prism through which the light of the Old comes to Christianity. For Judaism, the rabbinic corpus serves the same function. The corpus of the New Testament is well known.
The distinctive corpus of rabbinical Judaism, the so-called oral Torah, is less well known, but it is the star that guides Jews to the revelation of Sinai, the Torah.