This book charts the shifting boundaries of Judaism from antiquity to the modern period in order to bring clarity to what scholars mean when they claim that ancient texts or groups are “within Judaism,” as well as exploring how rabbinic Jews, Christians, and Muslims have negotiated and renegotiated what Judaism is and is not in order to form their own identities. Belief in Jesus as the Messiah was seen as part of first-century Judaism, but by the fourth or fifth century, the boundaries had shifted and adherence to Jesus came to be seen as outside of Judaism. Resituating New Testament texts within first- or second-century Judaism is an historical exercise that may broaden our view of what Judaism looked like in the early centuries CE, but normatively these texts remain within Christianity because of their reception history. The historical “within Judaism” perspective, however, has the potential to challenge and reshape the theology of contemporary Christianity while at the same time the long-held consensus that belief in Jesus cannot belong within Judaism is again challenged by the modern Messianic Jewish movement.