This paper examines the strategies of Dutch VOC pastors in engaging with the established Islamic communities of the East Asian spice islands during the seventeenth century. It explores a twofold dynamic by distinguishing between missionary theory and practice. First, it analyses the prescriptive theory underpinning such encounters through a close reading of Justus Heurnius's manual Ad Indos Capessenda Admonitio (1618/1628), a pioneering Calvinist treatise on mission, composed before Heurnius own mission in Batavia. Second, the paper contrasts this theory with on-the-ground practice, investigating the actual linguistic and social adaptations made by pastors in the Indies. It analyses, for instance, the pragmatic adoption of the Arabic lexicon, such as using 'Allah' for God in the works of contemporaries like Ruyll.