The study of sectarianism in Islam and the study of Muslim-Christian relations are both sub-specialities attracting growing numbers of scholars in Islamic studies. Rarely, though, are these two fields put into direct conversation with each other. In this work, Steven Gertz brings the two together to ask how the Sunni-Shi'a divide in Islam impacts Muslim relationships with Christians. Do tensions within Islam do more to help Muslim relationships with Christians, or harm them? Gertz goes about answering this through a historical study of the Fatimid caliphate in Palestine and Egypt during the fourth/tenth and fifth/eleventh centuries. He specifically works to understand how Fatimid religious principles (ascertained through the study of law) and politics (ascertained through the study of history) impacted Christians in light of Fatimid-Abbasid rivalry. In the process of doing so, he makes a valuable contribution to the study of Islamic religious identity formation as it concerns sectarianism within Islam and inter-religious relations with non-Muslims.