Academic texts of the twelfth century are regularly used by historians of medieval heresy for snippets of evidence on the “religion” of so-called heretical movements like the “Cathars.” The difficulties of using such evidence in this way are apparent when reading these academic texts in full, and considering them in their context: the theological disputations of the first universities. I will elsewhere write more about how developments in theology during the twelfth century affected the way the intellectual leaders of the church understood popular religion; and how the idea of there being heretical movements and the idea of removing them by inquisition both had the same origins. In this article I introduce some of these ideas by looking closely at only one part of one such academic text: Book One of the De fide catholica, of Alan of Lille.