Matthew Thiessen offers a nuanced study of the nature of Jewish thought with regard to Jewishness, circumcision, and conversion. Examining texts from the Hebrew Bible, Second Temple Judaism, and early Christianity, he gives a compelling account of the various forms of Judaism from which the early Christian movement arose. Beginning with analysis of the Hebrew Bible, Thiessen argues that there is no evidence that circumcision was considered to be a rite of conversion to Israelite religion. In fact, circumcision, particularly the infant circumcision practiced within Israelite and early Jewish society, excluded from the covenant those not properly descended from Abraham. In the Second Temple period, many Jews began to subscribe to a Jewishness that enabled Gentiles to become Jews. Other Jews found this definition of Jewishness problematic, and defended their own definition by reasserting a strictly genealogical conception of Jewish identity. As a result, some Gentiles who underwent conversion to Judaism in this period faced criticism because of their suspect genealogy. This examination of the way in which Jews in the Second Temple period perceived circumcision and conversion allows a deeper understanding of early Christianity. Contesting Conversion shows that careful attention to a definition of Jewishness that was based on genealogical descent has important implications for understanding the variegated nature of early Christian mission to the Gentiles in the first century c.e.