The article concerns the origin and development of the concept of "enthusiasm" in German philosophy, starting from the Reformation and ending with the philosophy of I. Kant. This concept is key to the philosophy and theology of the German Enlightenment. At the moment, the philosophical and even more theological thought of the Enlightenment in Germany is studied very little. At the same time, new studies of this period have appeared abroad in the last few decades. For the most part, these studies are aimed at identifying the key ideas of the German Enlightenment and clarifying the formation and development of some of them. “Enthusiasm” refers to one of the significant polemical ideas of this period, however, both in Russia and abroad, it has not been sufficiently studied. The article discusses the original meaning of this concept in the theology of Luther, who understood enthusiasm in the expanded meaning of any deviation from his own version of the Christian faith. Then, enthusiasm began to be understood more narrowly and associated with the predominance of attention to the inner sphere of religious experiences and the resulting neglect of the sphere of external religious practices. This understanding became more and more stronger over time and led to the fact that this concept began to be used to refer to religious movements of a mystical and quasi-mystical persuasion. Eventually, during the late German Enlightenment (second half of the 18th century), enthusiasm became practically synonymous with defining the trends of late radical pietism. Such an understanding of enthusiasm, in turn, finds its foundation in pietism itself, going back to the criticism of P. J. Spener, the founder of classical pietism, against the representatives of radical pietism. We also find a certain influence of this tradition of understanding reverie in Kant, who divides it into two types - religious and moral enthusiasm.