This thesis explores canonistic discussion of the inquisition of heretics during the first decades following the issuance of Excommunicamus , canon three of the 1215 Fourth Lateran Council. I have closely analyzed the canon, divided into eight sections plus introduction, in light of commentaries and the Ordinary Gloss on the canon, as well as comparing the text with the texts from which it was derived, particularly Ad Abolendam of Pope Lucius III (1184) and Vergentis in senium of Pope Innocent III (1199), as well as other decretals cited in the comments and glosses.

The study of the jurisprudence of heresy in the canon law literature leads to the three conclusions of this thesis: (1) heresy became articulated as a well-defined crime, with specific juridical elements like contumacia , and understood in the context of its relationship with other ecclesiastical crimes, together forming a body of crimes with distinctions in gravity (and appropriate treatments) and an increased inclusiveness of criminality (not just heretics, but helpers, innocent children, negligent secular leaders and laity are alike criminally treated as heretics). (2) The procedures leading to the condemnation of heresy become more defined and articulated, from being deprehensus as a suspectus or by evidentia (“convicted”), to sentence (declaration) of excommunication by ecclesiastical jurisdiction absent purgation and proper satisfaction, degradation and deposition in the case of clerics, and executio of punishment by the secular authorities, resulting in penalties and legal disabilities for the condemned, partially but not universally counterbalanced by concerns for equity. (3) There is an increasing secularization in the treatment of heresy, in the conceptualization of crime, in the methods of punishment, and in the increasing role of secular jurisdiction. These developments in the conceptualization of the treatment of heretics, before it becomes a more formalized Inquisition, come from the contributions of canonists who expounded on papal pronouncements in the extermination of heresy.