This article investigates the language of heresy in the Liber Pontificalis, a collection of papal biographies first compiled in the context of the sixth-century Symmachian/Laurentian Schism. More specifically, it considers the reported actions of the Bishops of Rome in opposition to the heresy of Manichaeism, focusing on the biographies of Gelasius (492–496), Symmachus (498–514), and Hormisdas (514–523). It is argued that these accounts are best understood as a heresiological motif employed by the compiler(s) of the Liber Pontificalis in order to emphasize and reinforce the authority and the legitimacy of those who were said to have opposed Manichaeism. This conclusion is suggested by the nature of the Liber Pontificalis itself, which was only one of a set of conflicting polemical sources that debated the recent past of Rome’s episcopacy. Moreover, as it was employed by Christian controversialists, “Manichean” had, by the sixth century, become an epithetical term of disapprobation rather than an accurate descriptor of a particular sect or system of belief. The depiction of the fight against Manichaeism in the Liber Pontificalis was thus a small part of a larger discursive project intended to represent the Roman Church—and Gelasius, Symmachus, and Hormisdas in particular—as the consummate enemies of heresy, the defenders of orthodoxy, and the locus of authentic and legitimate authority.