By analysing how Geo Widengren revised one of his former supervisor Tor Andræ’s more popular books in Swedish on Muhammad, I cast light on how Widengren dealt with the problem of personal belief in the study of religions. The aim of the chapter is to highlight both similarities as well as disruptions and breaks between the ‘master’ (Tor Andræ) and the ‘student’ (Geo Widengren). This comparison will also show that Widengren’s approach to the history of religions is different from his Swedish predecessors, Nathan Söderblom (1866–1931) and Tor Andræ. While both Söderblom and Andræ were thorough scholars, they were also in influenced by their Christian believes. As I will argue in this chapter, for Widengren it was essential to make a sharp distinction between personal beliefs and the academic study of religions. A peculiarity in Widengren’s attitude is to see the understanding of the meaning of a text in explaining the “historical background” of its terminology and content. “To clarify and understand” means: to see the influences behind. For Andrae the point is the function of the texts, their sense for their first audience (in this case the prophet’s companions) and in what ways this has a relevance for the general concept of human religiosity. For Widengren, the theological “function” of the text was of less importance.