In the Commentatio the 22-year-old Kuyper not only describes Calvin’s and a Lasco’s concepts of the Church, but also discusses them in the light of the Gospel. The Commentatio marks the beginning of modern a Lasco studies. The work also offers the initial impetus for the idea with which Kuyper would later exert great influence on Dutch nation and society: the Church as a free, democratic society of Christians, which manifests itself as a living organism in all spheres of life. The text, which has never been published before, is accompanied by historical and philological introductions, annotations, and comprehensive registers, and throws surprising new light on the origins of Kuyper’s ideas. Moreover, this source edition is important for the study of nineteenth-century Reformation research.