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.

This chapter offers a critical narrative of the development of Arianism as a heresy from the fourth to the sixth century. It explores the changing meanings of the heresiological label, and the political and ecclesiastical contexts in which it was deployed, from the origins of controversy between Arius and Alexander in Alexandria through to the barbarian successor kingdoms of the post-imperial West.

Even by the standards of medieval heresy, the history of the Bogomils is notable for a lack of sources. This chapter bases its analysis on the long account of Kosmas Presbyter of Pop Bogomil and his followers in tenth-century Bulgaria. Understanding this helps illuminate less informative references and points to a persistent dualist tradition in the Balkans and Byzantium, well capable of exporting organisation and doctrine.

The anthropology and soteriology of western Christianity were radically reinterpreted in the fifth century CE by Augustine of Hippo, who constructed a fictional ‘Pelagianism’ to delegitimise opposition to his new theology of original sin, an absolutist account of prevenient grace, and predestination interpreted as preordainment. This chapter gives an outline of the issues involved in this attempt to relocate orthodoxy, the course of events relating to Pelagius and his defence of eastern ascetic Christianity, and the afterlife of controversy over this new account of the anthropology and soteriology of Christianity.

The iconoclast refusal to accept the use of images, particularly of Christ, in public and private worship in Byzantium, is often condemned as a heretical movement. But its emphasis on symbolic representations (the Cross and the Eucharist) and on spiritual veneration was essentially a reform of potentially idolatrous church practice.

Whenever the story of lollardy has been told, that story has been shaped by the prevailing historical, theological and intellectual climate. This chapter surveys how lollardy has been narrated, looking at the terminology, people and communities, practices and texts, and beliefs associated with the lollard or Wycliffite movement of late medieval England.

This chapter provides an overview of the Nestorian controversy, including the background and aftermath of the Councils of Ephesus (431) and Chalcedon (451). Additionally, the chapter traces the distinct ecclesiological trajectories that emerged from these Christological debates.

The chapter deals with the history of the Hussite revolution in the first half of the fifteenth century in Bohemia. It focuses on the discourse of heresy, which underwent significant differentiation in the late Middle Ages as political conditions became increasingly complex.

This chapter surveys different ways in which ‘heresy’ has been conceptualised by a variety of writers, both within the periods in which it arose and in later centuries. Tracing a number of different inflections to the charge of heresy, the chapter suggests that we might see it not only as constructed by orthodox authority but as a means by which ‘authority’ itself is reaffirmed; and in conclusion suggests some ways through which modern historians might then reconceptualise their search for ‘dissent’ in past times.

The chapter traces the emergence of the notion of ‘gnostics’ as a classification, from originating confusion in the use of this language by ancient Christian anti-heresy writers to ongoing debates over this category among modern scholars. Progress in understanding the relevant ancient sources may benefit from analyses of specific themes and features in individual texts but without the encumbrance of disputes over a troubled classification (‘gnosticism’) that ultimately is a relic of ancient heresiology.

The Waldensians began inside the church in the 1170s, were excommunicated, went underground and survived into the sixteenth century. In our efforts to get at their past reality, how far can we penetrate the texts about them produced in the Middle Ages by a persecuting church, during the Reformation and Counter-Reformation by polemicists and in modern times by academics modelling them according to the latest intellectual fashions?

Were ‘the Cathars’ a medieval reality, an actual religious dissidence entertained by an organised group of people, or are they a mere figment of medieval churchmen and modern historians? In the last two decades, a fierce scholarly controversy has developed around this question. The present chapter introduces the reader to both sides of the argument.

This chapter introduces the main themes and scope of the volume, including discussing the origin of the concept of ‘heresy’, as well as outlining what aspects of it will and will not form the focus of the following chapters. It then provides a summary of the division of the volume into two parts and the particular topics and case studies contained in each.