The subject of this study is the establishment of the term ‘Protestanten’ as a general term for the opponents of the Roman Church in early modern history. The origin of the term stems from the protest of certain estates at the Diet in Speyer 1529 against the cancellation of the limited concession to proceed with the reformation which had been made in Speyer in 1526. Previous studies observed that the term was primarily used in documents related to the Diet. In addition, the first historian of the Reformation, Johannes Sleidanus, employed it in his De statu religionis et reipublicae … commentarii, published in 1555. Soon authors in Western Europe spoke of ‘Protestants’ when they wrote about the Reformation. In the German Empire however, the term ‘Protestanten/Protestierende’ was not used as a term for the opponents of the Roman Church. The reason for this was the growing conflict between the Lutheran and Reformed Protestants. From a Lutheran point of view, the term blurred the differences between Lutheran and Reformed theology, something which in their eyes was to be avoided. In contrast, the theologians in the Electoral Palatinate, where the Heidelberg Catechism of 1563 taught Reformed doctrines, tried to emphasise their accordance with the Augsburg Confession. This was essential, because the peace of Augsburg in 1555 only protected adherents to this confession. The main argument was that Melanchthon's 1540 version of the confession gave room to Reformed interpretation of the doctrine of the lord's supper. The author of this study shows that in this context the term ‘Protestanten/Protestierende’ was widely used to integrate Reformed and Lutheran theologians. The main authors of the Palatinate irenic theology, Zacharias Ursinus and David Pareus, used the term to underline the communality of Lutheran and Reformed theology (pp. 19–54). Later, other Reformed theologians such as Wilhelm Zepper and Christoph Pezel did the same (pp. 55–90). Meanwhile, Lutheran theologians of the seventeenth century such as Johann Georg Sigwart, Leonhard Hutter, Matthias Hoe von Hoenegg, Abraham Calov or Johann Conrad Dannhauer argued that Reformed theologians did not have the right to be called Protestants (pp. 91–158). The Lutheran use of the term as an integrative term for Lutheran and Reformed started with the reception of Georg Calixt's irenic doctrine of the fundamental articles in the second half of the seventeenth century (pp. 159–92). This study analyses a multitude of Lutheran authors who employed the term as an integrative term, for example Samuel Pufendorf in his Jus feciale divinum, first published in 1695 (pp. 193–201). In these decades, Pietists like Philipp Jakob Spener already spoke of Protestants in an irenic sense (pp. 201–12). In Gottfried Arnold's Unpartheyische[r] Kirchen- und Ketzer-Historie, first published in 1699, the term achieved a critical purpose (pp. 212–56). Arnold used it especially when he criticised Lutheran and Reformed theologians for having betrayed the communion of the early Reformation. The importance of this study lies in that it demonstrates the reasons not only for the use of the term ‘Protestanten/Protestierende’, but also for its non-use.