This project investigates the role and usage of heresy accusations in the Grand Duchy of Lithuania (GDL) between the 13th and early 16th centuries, a period marked by Christianisation, political consolidation, and religious diversity. Despite the absence of formal inquisitorial trials, accusations of haereticus and its derivatives played a key role in framing dissent and otherness. The study explores four key contexts: the persistence of pagan practices and folk magic, where clerical rhetoric often equated local beliefs with devil worship; political rivalries, in which “heretic” functioned as a polemical weapon and justification for conflict; engagement with Hussite dissent, reflecting tensions between popular religious movements and elite strategies of power; and the treatment of Orthodox Ruthenians, whose faith was reframed as heretical within broader attempts at Catholic-Orthodox union. Methodologically, the project applies discourse analysis, sermon studies, and recent historiographical approaches to legal and ecclesiastical sources. By examining heresy as both a semantic and political construct rather than solely doctrinal deviance, the research highlights the complex intersections of religion, identity, and authority in the proto-Reformation GDL, offering new insights into the dynamics of Christianisation and interconfessional relations in late medieval Eastern Europe