John of Damascus (c. 650-750) is one of the best known Christian theologians of the eighth century. Most of his theological works have been the subject of prolonged and repeated scrutiny, with a recent renewal of interest in his thinking among scholars. Although little is known ofthe details of John's life, his well-attested upbringing in Damascus and later position as an administrator for thy Caliph placed him in a unique position to witness the early development of the religion today known as Islam. This thesis seeks to free earlier conceptions of John's treatise, and explain how John could understand Islam as a 'heresy', when such a term of application to Islam would not have occurred to most of his predecessors, peers, or successors in the Byzantine theological tradition, nor to modem scholars of heresy. By considering early Christian heresiology in its various forms and contexts, the first half of the thesis shows how it was possible for John to conceptualize Islam within the category of 'heresy', and that contrary to earlier assumptions, having done so did not require John to think of Islam as a deviant form of Christianity. Further earlier conceptions of the work are normally made against the background of false assumptions: that a highly developed Islam existed in the eighth century when John wrote his treatise and that he considered Islam a 'Christian heresy'. By reconsidering his text, and situating John in his historical and theological context, it is possible to show that we are not in a position to say with certainty how accurate John was in his observations. At the same time, there is no reason to consider his treatise other than an accurate historical reflection of events as he perceived them in his time. 11