This chapter explores how the ancient literary and philosophical dialogue form maintained its relevance as a tool of cultural and religious identity formation and competition. It addresses the ongoing scholarly discussion regarding the scope for dialogue in the face of rising authoritarianism and dogmatism in late antiquity.

Heresiological catalogues are one of the most distinctive forms of literature concerning heresy and have also traditionally been one of the most difficult to interpret. This chapter traces the development of this form of writing, focusing on the most productive period in late antiquity, and explores how much such works were intended to be employed in active ‘heresy-hunting’ and what other purposes they might have had, including increasing the authority and status of their authors.

Numerous Christian authors from the seventh century to the twenty-first have classified Islam as a Christian heresy. Writers from John of Damascus in the seventh century to Hilaire Belloc in the twentieth have seen Muhammad as a heresiarch who forged a heresy based on elements of Judaism, Christianity and Paganism. Comparison between Muhammad and earlier heresiarchs (such as Arius or Nestorius) allowed Christian authors to denigrate and dismiss Islamic doctrine. Such comparison also facilitated the denunciation of new heretics closer to home: Luther and Calvin, for Catholic polemicists of the sixteenth century, or on the contrary the ‘papists’, for Protestant writers.

This chapter examines the concept of heresy through the growth of papal decretals, canon law and conciliar legislation during the High and Late Middle Ages (eleventh to fifteenth centuries). It explores the growth of medieval heresy during a period of burgeoning papal power and examines how popes both reacted to and constructed ideas of heresy and heretics.

This chapter examines the reciprocal accusations of heresy between the Byzantine East and Latin West: such accusations were only sporadically made during the first millennium but increased exponentially from the twelfth century onwards on the Byzantine side and, to a lesser extent, from the thirteenth century on the Latin side (where they were generally subsumed under the accusation of schism or ‘disobedience’). The notions of ‘heretic’ and ‘schismatic’, which ought to be distinct and precisely defined according to canon law, increasingly overlapped within the polemical discourse and were collectively applied to the opposite population, in a process of construction of religious otherness.

The documentation related to inquisitorial practice included trial transcripts, consultations, sentences, manuals and accounts. But while this documentation was a key ingredient in inquisition’s power, the relationship between these sources and our knowledge of medieval heresy is complex.

Unlike many other forms of ‘heresy’, Manichaeism is not a polemical construct but an independent and organised church within the Christian tradition originating in Mesopotamia and Iran in the third century CE. It had its own heresiology against other communities such as the western Catholic-Orthodox church, identifying key points of difference and deviance from the ‘faithful and true Christianity’ of Manichaean doctrine and practice.