The study examines the elements of the ecclesiastical polity of the Transylvanian prince Gabriel Bethlen. Gabriel Bethlen is regarded by historical tradition as a “Reformed prince” and an example of early modern religious tolerance. This paper explores the roots of this professed Reformed identity and the plan for the creation of a Calvinist state. In particular, it describes Gabriel Bethlen’s close relationship with Heidelberg and shows how and to what extent this small Calvinist state became a model for Gabriel Bethlen, a model that was also reflected in his construction projects. The study also discusses the elements of Gabriel Bethlen’s ecclesiastical polity that made it difficult to implement his plan for a Reformed state, as well as the long-term consequences of this.