The decline of “fanaticism” in eighteenth-century Germany, a myth propagated by self-proclaimed proponents of Enlightenment, continues to shape historians’ representations of the ascendancy of “religious” Enlightenment. To discredit this myth and suggest a means of replacing it, this essay departs from the conventional attention to university theology as a history of ideas and proposes adding a book-historical perspective. Its focus is the German Pietist theologian Joachim Lange (1670–1744). Condemned by critics as a “fanatic” by virtue of his alleged intellectual kinship with French Reformed theologian Pierre Poiret (1646–1719), Lange is best known today for his vehement and ultimately ineffectual opposition to Enlightenment’s theological standard-bearers at the University of Halle. But Lange’s kinship with Poiret was only partial, and the stark contrast between the careers of two of Lange’s textbooks reveals that although his theological star was falling by the 1730s, elements of Lange’s ostensibly outmoded theology continued to find an audience into the nineteenth century, through the enormous commercial success of his Latin grammar.