This article provides a new perspective on the discussion of heresy from one of the most influential canonical-jurisprudential commentaries of the Middle Ages: Bernard of Parma's Glossa ordinaria to Pope Gregory IX's Decretales (commonly known as the Liber extra). Based on an analysis of Bernard's legal glosses, with special emphasis on his citation of Roman and canon law traditions, I argue that the often-overlooked Glossa ordinaria provides scholars a unique window into medieval conceptions of heresy, jurisprudence, and ecclesiastical-legal practice. This study demonstrates that this important mid-thirteenth-century legal-educational text not only reoriented the canonical definition of heretics toward an emphasis on sects rather than individuals, but, differing from the contemporary, often severe papal and conciliar rulings against heretics, also stressed the centrality of mercy and temperance in how heretics should be treated by the ecclesiastical court. The Glossa ordinaria, as this article discusses, might have served as an intellectual force that could have counter-balanced the overzealousness of emerging inquisitors in an age of intensifying repression of heretics.