How can we study religious conversion in the early Islamic period? This paper presents the findings of a study on religious conversion in Greater Syria (Bilad al-Sham) and northern Iraq (al-Jazira) from roughly 640 to 850 CE (20-235 AH). It analyzes textual data on conversion written in Arabic and Syriac, aggregated from early chronicles, hadith collections, martyrologies, apocalypses, polemical treatises and legal texts, to identify the way conversion is discussed in general and within each genre. It also examines topics discussed in tandem with conversion to better understand the way in which it was perceived by scholars writing in Syriac and Arabic during the first two hundred years after the Muslim conquest of Bilad al-Sham and al-Jazira. While reliable demographic figures are impossible to come by during the early Islamic period, this study finds that the way conversion was described in both Syriac and Arabic texts changed over the course of the first two hundred years of Muslim rule of these regions. Writing on conversion increased in both sophistication and in polemical tone, perhaps as conversion became more common and thus increased in stakes for writers of different faiths. Muslim belief also became more distinct from other monotheistic faiths over the course of the seventh century CE in particular, which allowed writers to clarify religious differences and set defined boundaries between these faiths. A concern with boundary demarcation between faiths is noted throughout the era, though inversely. While Muslim writers were initially most preoccupied with false converts, or converts maintaining non-Muslim beliefs, this concern was gradually superseded in the textual evidence by Christian writers, for example, who become increasingly worried about individual Muslim beliefs and practices taking hold among their constituents. It must be noted that during this period, both Muslim and non-Muslim writers of various genres frequently associated false conversions with military service or payment of taxes. Possible interpretations of this finding and other conclusions will be presented, as well as potential avenues of further study.