Matthew Thiessen offers a nuanced and wide-ranging study of the nature of Jewish thought on 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 definition of Jewishness that enabled Gentiles to become Jews. Other Jews, such as the author of Jubilees, found this definition problematic, 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.Thiessen's 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 crucial implications for understanding the variegated nature of early Christian mission to the Gentiles in the first century C.E.