In 16th-century Europe, few topics were so highly contested as the unity of the Church. In this context, the Italian Reformed theologian Girolamo Zanchi (1516-1590) presented a distinctive account of ecclesial unity that, while often expressed amid conflict, nevertheless offered a rich theological vision of the one Body of Christ. This monograph explores that vision. It draws upon the full scope of Zanchi’s academic output to trace a Trinitarian ecclesiology that was biblically grounded and reflective of Reformed confessional commitments, yet also informed by a diverse and harmonized array of patristic (especially Augustinian), medieval, and Neoplatonic elements. From this study, fresh insights emerge into Zanchi’s intellectual range, the independence of his exegetical judgments, and the breadth of his engagement with the theological and philosophical traditions that preceded him.