Martin Luther’s adversarial conception of Jewish religious legalism ironically was a major factor informing the self-perception of modern European Jewish consciousness. This chapter uses the theme of religious legalism to address the process in which belonging to the law became a theological stance and subsequently a core component of religious identity. It argues that the characterization of Judaism as a law-based religion is a modern phenomenon that was propelled and accelerated by an ideological discourse that aimed to systematically differentiate between religious affiliations and identities and to map the fundamental differences between religions. The argumentation in this chapter is based on a historical survey of the interplay of law, religion, and identity in the late ancient Judeo-Hellenic world, the medieval Judeo-Arabic milieu, and post-Reformation Europe.