The Glossa ordinaria compiled by Bernard of Parma (d. 1266) on Pope Gregory IX’s 1234 Decretales, commonly known as the Liber extra, is among the most influential canon law commentaries during the High and Late Middle Ages. Interrogating this source, this dissertation examines the legal status of selected marginal religious groups in medieval Europe—apostates, heretics, Jews, Muslims, and practitioners of magic. Soon after its emergence, Bernard’s Glossa was studied by law school students—that is, future Church judges, lawyers, inquisitors, and even popes—from the mid-thirteenth century on, and was the standard commentary copied into the margins of manuscripts of the Decretales. Yet, modern scholarship ignores this source almost entirely. This study treats this issue through transcription, translation, comparison, and analysis of texts from selected medieval manuscripts of the Decretales and the Glossa, including the earliest surviving exemplars (c. 1240). It explicates the Romano-canonical judicial terminology and principles employed by the Glossa. Furthermore, it scrutinizes the Glossa’s manner of using legal allegations and tracks the excerpts which it inherits from commentarial literature. Finally, it examines how the Glossa treats the selected marginal religious groups, and thus uncovers how this source can serve as a window for us into medieval society from the perspective of the learned or academic law. More broadly, this work contributes to a fuller understanding of the development of medieval canonical science, the operation of the ecclesiastical-legal system, and the mechanism through which the institutional Church defined its own religious boundaries.