During the early modern period, European knowledge of Islam expanded markedly through the field-based activities of Christian missionaries operating in regions of the Islamic world. Recent scholarship has shown how some of these missionaries transformed the experiential knowledge acquired over extended periods of mission into substantial textual productions that circulated widely in multiple European languages. In these works, enduring patterns of religious polemic coexisted with extensive ethnographic and doctrinal material on Muslim beliefs and practices, reflecting the complex entanglement of confessional agendas and early modern knowledge production. This paper examines the conceptual translation of the doctrine of tawḥīd in the writings of seventeenth-century Catholic missionaries active in Arabic-speaking regions of the Levant, whose works were shaped by first-hand knowledge of Islamic rituals and devotional practices, notably the Jesuit Michel Nau and the Capuchin Michel Febvre. Fluent in Arabic and other Oriental languages, both authors produced wide-ranging works on Islam addressed to a European readership, as well as more concise handbooks designed as missionary tools of religious controversy. By comparing these different textual genres, the paper analyzes how the doctrine of tawḥīd was reframed in relation to the specific aims of missionary writing and the distinct audiences to which these texts were addressed.