This book examines ancient Jewish thought with regard to Jewish identity construction, circumcision, and conversion. It argues that there is no evidence in the Hebrew Bible that circumcision was considered to be a rite of conversion to Israelite religion. The infant circumcision that was practiced within Israelite and early Jewish society, and was demanded by Genesis 17, excluded from the covenant those not properly descended from Abraham. In the Second Temple period many Jews did begin to conceive of Jewishness in terms that enabled Gentiles to become Jews. Nonetheless, some Jews, especially the author of Jubilees, found this definition of Jewishness problematic, and they defended the borders of Jewishness by reasserting a strictly genealogical conception of Jewish identity. Consequently, some Gentiles who underwent conversion to Judaism in this period faced criticism because of their suspect ethnicity. Second Temple Jewish sources record such exclusion with regard to the Herodians, who were Idumean converts. This examination of the way in which Jews in the Second Temple period perceived circumcision and conversion provides a better understanding of early Christianity as the book of Acts portrays it. The final chapter demonstrates how careful attention to a definition of Jewishness that was based on genealogical descent has implications for understanding the disputes over the early Christian mission to the Gentiles.