An Anxious Inheritance reveals the tensions between the early framers of Islam and the ever-expandable category of non-Muslims. Examining the encounter with these religious others, and showing how the Qur'an functioned as both a script to understand them and a map to classify them, this study traces the key role that these religious others played in what would ultimately emerge as (Sunni) orthodoxy. This orthodoxy would appear to be the natural outgrowth of the Prophet Muhammad’s preaching, but it ultimately amounted to little more than a retroactive projection of later ideas onto the earliest period. Non-Muslims (among them Christians, Jews, Zoroastrians) and the wrong kinds of Muslims (e.g., the Shi'a) became integral–by virtue of their perceived stubbornness, infidelity, heresy, or the like–to the understanding of what true religion was not and, just as importantly, what it should be. These non-Muslims were rarely real individuals or groups; rather, they functioned as textual foils that could be conveniently orchestrated, and ultimately controlled, to facilitate Muslim self-definition. Without such religious others proper belief could, quite literally, not be articulated. Shedding new light on the early history of Islam, while also problematizing the binary of orthodoxy/heresy in the study of religion, An Anxious Inheritance makes significant contributions to a number of diverse academic fields.