Only rarely do medieval anti-heresy writings contain discussions about Christian communities in the East. Greeks, Armenians, Jacobites, Georgians or Nubians are usually absent from Western anti-heresy treatises, even when these are concerned with the Oriental sects from late antiquity drawn from Augustine and their subsequent updates. Indeed, the pragmatic concern to defeat those sects which seemed to be threats to the Latin West was the obvious mission inspiring the authors of most anti-heretical polemic.

Yet we do have a few treatises, directed against a great variety of errors, which also contain refutations of the errors of various Eastern Christian groups. The mid-thirteenth century Tractatus fidei contra diversos errores (Treatise of faith against various errors) by Benedict of Alignan, for example, engages with the errors of Greeks and Armenians, while also including discussions of Muslim beliefs and Jewish practices. Similarly, various anti-heresy treatises compiled in the fourteenth century include lists of errors of the Eastern Christians: they comprise Guido Terreni's Summa de haeresibus et earum confutationibus (Summa of Heresies and their Refutations), Alvarus Pelagius's Collyrium fidei contra haereses (Eye-salve of Faith against Heresies) and the renowned Directorium inquisitorum (Directive for Inquisitors) by the Dominican inquisitor Nicholas Eymerich. The two latter works include rebuttals of the Greeks only, whereas Guido's Summa contains lengthy descriptions and refutations of the errors of Greeks, Armenians, Georgians and Jacobites, with a special focus on the Greeks and Armenians. In addition, Guido engages in polemical argumentation against the doctrines and practices of the Jews, while one copy of the Summa contains a further fragment referring to twenty-five errores Sarracenorum, whose attribution to the same author has however been disputed.

How did Eastern Christians’ errors fit into anti-heresy treatises? And what do discussions about these groups reveal about learned approaches to heresy in the later Middle Ages? This essay focuses on what Guido Terreni and Alvarus Pelagius had to say against heresy in two comprehensive summae that were produced in the intellectual milieu of the Avignon court. I shall consider these treatises and look comparatively at the treatises by Benedict of Alignan and Nicholas Eymerich, in order to examine the place occupied by Eastern Christianity in the construction of a universal knowledge of heresy in the later Middle Ages.