This book is a sequel to A New Science: The Discovery of Religion in an Age of Reason, where I analyzed new intellectual approaches to religion in early modernity, from the sixteenth to the eighteenth century.1 In the present work, I study some major aspects of the scholarly study of religion during the long nineteenth century. More precisely, I seek here to understand the implications, in a secular age, which was also the formative period of the new discipline, of a major paradigm shift. The nineteenth century witnessed the transformation of the taxonomy of religions. According to the traditional model, in place since late antiquity, Judaism, Christianity, and Islam were cognate religions, all stemming from the biblical patriarch Abraham’s discovery of monotheism. This model was largely discarded during the Enlightenment, and would be later replaced by a new one, according to which Christianity, the religion of Europe, essentially belonged to a postulated family of the Aryan, or Indo-European religions, while Judaism and Islam were identified as Semitic religions....