This Element examines the forms of Arabic used by Christians in the early Islamic period in theological treatises and Arabic Bible translations. It argues that linguistic analysis of these texts not only clarifies the nature of early Islamic Arabic but also sheds new light on Christian institutional and intellectual culture. Focusing on nominal case, verbal mood, and gender and number agreement, the study challenges the common view that Christian authors wrote either flawed Classical Arabic or in a substandard register. Instead, it shows that their Arabic was typical of the early Islamic period. The Element also identifies differences in linguistic choices between theological treatises and biblical translations. After the Muslim conquests, Arabic was the language appropriate to both genres. The Element argues that Christians deftly and creatively adapted Arabic writing to their literary activities, in language appropriate to their different audiences.