As a regional denominational conflict, the "diplomatic dispute of Cologne" was of relevance for the state-church law of the Holy Roman Empire as a whole. The present study analyses how Colognes Reformed congregations sought to improve their status in the Catholic imperial city by making use of the Prussian kings right to resident ambassadors. The denominational conflict refers to an exciting, yet unterstudied period of Colognes history. Drawing to extensive source work, the study reconstructs the course of historical events, taking into account denominational perspectives. Of particular interest is the mediation process which defused the conflict. Emerging rights of diplomatic ambassadors are delineated in their development. Here, relevant raison detat provided a gateway for the confessionalisation of law.