This article examines the Halle-Herrnhut controversy from 1727 to 1737 by foregrounding the role of the Moravian mission program and the spiritual agency of lay believers in shaping ecclesiastical politics. Moving beyond the conventional focus on theological disputes and personal animosities between the Franckes and Count Zinzendorf, the analysis will examine how Moravian missionaries and common believers were represented and rhetorically mobilized within a transatlantic correspondence network as symbols of ecclesiastical disruption and spiritual populism. Drawing on unpublished German-language letters exchanged among Halle Pietists and their allies, the article reveals how concerns about denominational proximity, spiritual authority, and institutional control were rhetorically projected onto Moravian expansionary initiatives. The controversy, which unfolded across Europe, British North America, and as far east as India, was less about doctrinal precision than about reputational anxieties and competing visions of Protestant community. Moravian believers, who were often lay artisans, refugees, and itinerant missionaries, emerged as symbolic figures whose perceived spiritual agency threatened the coherence of Halle’s program of reform. By tracing how elite actors invoked these believers in polemical discourse, the article offers a new categorical lens for understanding the politics of mission, conversion, and ecclesiology in the eighteenth-century Protestant Atlantic world.