Rationalism is a cornerstone of the modern Occident’s conception of itself. However, as shown by the post-colonial reading of the term found in Max Weber’s comparative studies of religion, the concept of rationalism is also an instrument for hierarchization and essentialization. Rationalism is by no means the privilege of modern Western civilization, as shown by this study on the theological history of Ismailism. The study’s decolonized perspective furnishes a new approach for the historical comparison of religions.