In 2019, news outlets around the world reported the publishing of an open letter by a small group of conservative Catholic figures in which they criticised some of the views and policies of Pope Francis.Footnote ¹ One of the most prominent elements in these stories was the charge of ‘heresy’ against the 82-year-old pontiff, enabling journalists to make liberal use of the phrase ‘is the Pope a Catholic?’. Accusations of heresy are, of course, now very rare among Christians and for many readers of this news story the term must have seemed comically archaic, conjuring up lurid images of ‘medieval’ inquisitions.Footnote ² Nonetheless, it serves as a reminder of the significance and power of a concept that played a central role in Christian discourse for many centuries and has still not completely disappeared even in the twenty-first century.Footnote ³ Within this long history, two periods stand out particularly prominently for the emergence of new heresies and the development of mechanisms for their description and suppression: late antiquity, especially after the beginning of imperial support for Christianity in the early fourth century; and the later middle ages, most notably in western Europe. Moreover, as many of the chapters in this volume demonstrate, the same heretical names and categories, such as ‘Manichaean’ or ‘Arian’, often recurred throughout the centuries following their original appearance, enjoying renewed power in different contexts and conflicts.

The original development of the concept of ‘heresy’ within Christianity has been the subject of significant debate. The ancient Greek term hairesis meant ‘choice’ and was employed in the classical world to describe the different ‘schools’ or ‘sects’ of philosophers and doctors in a more neutral sense, only gradually coming to acquire its more familiar pejorative meaning in Christian literature.Footnote ⁴ For example, hairesis appears nine times within the New Testament, with six of these being in Acts, but this does not mean that all, or even any, of these uses correspond to the later developed notion of heresy, even if they sometimes carry a negative sense of factionalism.Footnote ⁵ More importantly, the emergence of a notion of ‘heresy’, along with its counterpart ‘orthodoxy’, certainly does not need to have coincided with the earliest employment of this term with such a meaning.Footnote ⁶ Robert Royalty has argued that such a discourse is already present in the New Testament and several other texts which emerged from the context of Second Temple Judaism, defining this ‘cluster of rhetorical forms’ as comprising five key elements: group membership or identity based on belief; an ‘apocalyptic eschatological worldview’ associating opponents with demonic influence; an emphasis on the transmission and preservation of tradition; a doxographical approach to the views of others; and the construction of links between all enemies to create a single, opposed ‘Other’.Footnote ⁷ Such elements can certainly be found in a number of these texts and undoubtedly contributed to later heresiological writings, but it is unclear whether they can be said to have constituted ‘heresy’ in themselves. A more cautious interpretation is provided by Daniel Boyarin in his analysis of this phenomenon in both Rabbinic Judaism and early Christianity, arguing that while ‘elements that would eventually constitute heresiology were already in place before their aggregation into a discourse of heresiology’ the true emergence of concepts of orthodoxy and heresy also required another rhetorical move of self-identification ‘from representing themselves as the embattled group that has the truth (sect) to the always/already there possessors of the truth that others are attempting to suborn (orthodoxy/“church”)’.Footnote ⁸ As such, the debate is less about when many of these rhetorical methods of ‘othering’ first emerged and more about which other factors needed to be present before they could be said to have coalesced into our recognisable concept of ‘heresy’. It therefore also intersects with wider debates about the development of the ‘institutional church’ in early Christianity, as well as the question of whether it is possible to identify a ‘proto-orthodox’ group which employed the rhetoric of ‘heresy’ to demonise its opponents.Footnote ⁹

For most scholars, the second century is regarded as an important moment in the history of heresy – and the textual genre of ‘heresiology’ – especially due to the writings of figures such as Justin Martyr and Irenaeus of Lyon.Footnote ¹⁰ As both Richard Flower and Michael A. Williams describe in their chapters, these texts played key roles in promoting the highly pejorative use of the term hairesis and also in creating a sense of different ‘sects’ and the relationships between them, most notably for a variety of ‘Gnostic’ groups.Footnote ¹¹ Their paradigms were followed by other authors who employed these methods to condemn practices and theological positions that they regarded as being outside the acceptable bounds of the religion. Strikingly, the possibility of appealing to ‘secular’ power to intervene in intra-Christian disputes can also already be glimpsed in the second century, coming into being alongside the concept of heresy itself. Justin Martyr, when complaining to the Roman emperor in his First Apology about the persecution of Christians by the authorities, argues that it is his ‘heretical’ opponents who ought to receive such punishments, rather than the faithful.Footnote ¹² As the number of Christians grew and more disagreements developed, the forms and quantity of literature concerning heresy also increased dramatically, especially after Christianity – or, rather, particular interpretations of it – received recognition and support from a succession of rulers, beginning with Roman emperors from Constantine (r. 306–37) onwards and continuing with both the various successor states in western Europe and North Africa and also the remaining Roman territories in the eastern Mediterranean, often referred to in modern literature as the ‘Byzantine empire’.

The advent of Christian rulers also brought greater potential for the secular authorities to take action against individuals and groups deemed to be heretical. Constantine would go on to be remembered as a persecutor by the Donatists in North Africa, while his son Constantius II (r. 337–61) was on the receiving end of the same accusation from Nicene Christians, often in forceful and lurid terms. Nonetheless, the integration of ‘heresy’ as a formal crime into Roman legislation was a gradual process, gathering pace from the late fourth and early fifth centuries onwards, as evidenced by the laws gathered in chapter 16.5 of the Theodosian Code, which was promulgated in 438.Footnote ¹³ The kingdoms that emerged from the end of the western Roman empire inherited parts of this legislative tradition and their rulers varied in their enthusiasm for taking action against heretics, but in the Byzantine lands legal restrictions continued to develop and harden, including under the emperor Justinian (r. 527–65), who was heavily involved in a number of theological disputes.Footnote ¹⁴ In the late Roman period, the death penalty was not prescribed for heresy, but, despite opposition from some ecclesiastical figures, it gradually became usual, first in the Byzantine empire and then in numerous western realms, especially with the expansion of anti-heretical concerns in this region during the High Middle Ages, which is explored in the later chapters of this volume.Footnote ¹⁵ Under the influence of both the legacy of Roman legislation and the growing body of canon law, linked with increasingly strident ecclesiastical attempts to deal with heresy, rulers of individual states came to enact their own secular legislation, responding to the particular circumstances and concerns within their domains.Footnote ¹⁶ Moreover, as John H. Arnold describes in Chapter 1, during the course of the medieval period, theologians, ecclesiastical figures and government officials created a range of different ways of defining ‘heresy’ and methods for identifying and combatting beliefs, practices and individuals which they deemed to be heretical. Although these centuries witnessed a multitude of differing conceptions of exactly who and what fell into this category, there was nonetheless a widespread view that heretical sects were distinct, real-life groups of people who had departed from the stable orthodox traditions of true Christians and posed a threat that needed to be addressed, sometimes through violent repression.

This traditional ‘essentialist’ view of orthodoxy and heresy has, however, been challenged and largely overturned by modern scholarly approaches to the subject. The work of Walter Bauer has been particularly important to this development for the beginning of the period covered by this volume, since it posited a model of early Christianity in which multiple ‘Christianities’ competed for supporters and acceptance, rather than there being a single orthodoxy that was assailed by a succession of ideas and movements that were condemned as ‘heresies’.Footnote ¹⁷ Around the same time, Herbert Grundmann made a very significant and influential contribution to the study of heresy by arguing that the high medieval period witnessed widespread movements aimed at recreating the ‘apostolic life’, some elements of which became accepted as orthodox while others ended up being classified as ‘heretical sects’.Footnote ¹⁸ More recently, scholars have nuanced and developed these approaches by moving away from the belief that the history of Christianity has been marked by the existence of a number of stable and clearly defined groups of believers who challenged each other for dominance. Instead, there is now a much greater focus on the practices and processes of ‘identity formation’ in early Christianity, looking at the shifting nature of identities and beliefs, so that, in the words of David Brakke, ‘the historian does not take for granted the existence of defined groups, but instead interrogates how ancient people sought to create, transform, and challenge religious communities and practices’.Footnote ¹⁹ Moreover, scholars of heresy in all periods have come to interrogate the reliability of the names for different ‘sects’ that have been transmitted to us, often by hostile literary traditions. This is partly because of a recognition that such labels have often been applied polemically, both to deny the title of ‘Christians’ to individuals and groups and also to link them with theological positions that they did not actually hold, particularly in the case of terms such as ‘Arian’, ‘Nestorian’ and ‘Pelagian’, as Robin Whelan, J. Edward Walters and Ali Bonner discuss in their chapters.Footnote ²⁰

Moreover, much scholarship now argues that many notable ‘heresies’ were not discrete religious movements and organisations of people with distinct identities and clear aims but rather were created by hostile, heresiological discourse, which assigned the status of ‘sects’ to a wide range of disparate ideas and phenomena, as well as to individuals who would not have recognised themselves as members of such a group. This approach has become extremely popular and fruitful in the study of a number of ‘heresies’ across the full chronological range covered by this volume, including ‘Gnosticism’, ‘Arianism’, ‘Pelagianism’, ‘Iconoclasm’, ‘Catharism’ and ‘Waldensianism’, as Michael A. Williams, Robin Whelan, Ali Bonner, Judith Herrin, Jörg Feuchter and Pete Biller explore in their respective chapters on these topics.Footnote ²¹ In addition, these trends have also led to the questioning and dismantling of many of the supposed links between different sects that appear in heresiologies and other anti-heretical literature. These developments, with their recognition that ‘heresies are made, not born’, have led to a significant increase in scholarship on the notion of ‘heresy’ as discourse, with particular emphasis being placed on its literary construction and the social and cultural contexts in which the concept was deployed.Footnote ²² As John H. Arnold cautions, however, historians must be careful about taking such approaches to the ‘constructed’ nature of heresy too far and thus erasing real and significant elements of disagreement and ‘dissent’.Footnote ²³ While some ‘sects’ which appear in our sources, such as Epiphanius of Salamis’ naked ‘Adamians’ from the fourth century, may have been products of an overactive heresiological imagination, we certainly cannot assume that the same is true of every heresy across two millennia of Christian history, as is emphasised in a number of chapters in this volume, especially those by Jörg Feuchter and Pete Biller on the Cathars and Waldenses respectively.Footnote ²⁴ Developments in scholarship have brought a number of important new insights and approaches for the study of ‘heresy’ across all periods, but it remains vitally important to assess each example of the phenomenon on its own individual terms.

Recent years have thus seen significant shifts in both our understanding and reconstruction of individual ‘heresies’ and also our approaches to the interpretation of historical material concerning ‘heresy’. Accounts of major developments and events can be found distributed across the major multivolume histories of these periods, such as The Cambridge Ancient History, The New Cambridge Medieval History and The Cambridge History of Christianity.Footnote ²⁵ There are also some excellent synoptic guides to the phenomenon across time and space, such as Christine Caldwell Ames’ Medieval Heresies: Christianity, Judaism, and Islam.Footnote ²⁶ All of the particular heresies or ‘sects’ discussed in The Cambridge Companion to Christian Heresy are the subjects of numerous monographs and articles. Any one of them deserves to have its own dedicated Handbook or Companion volume with chapters on a variety of different aspects of their history, beliefs and reception, and some, such as the Waldenses, Hussites and lollards, already do.Footnote ²⁷ This current volume, with its eighteen chapters spanning almost as many centuries, is therefore necessarily selective: it is a Companion to Christian heresy, not a History of it. More importantly, it is specifically about ‘heresy’ as a concept and a discourse rather than about episodes of theological disagreement and conflict in a more general sense. It is not intended as a comprehensive guide to all the different Christian ideas, groups and theologies that ultimately failed to achieve the status of ‘orthodoxy’, either when they first emerged or in the longer term: this is not The Cambridge Companion to Less Successful Christianities.

Instead, this volume is designed to cover the most important examples of the use of ‘heresy’ as a means of dividing, classifying and policing Christians, as well as the different methods employed for this purpose. It does, of course, provide information about the key people, events and theological disagreements involved in each dispute, sometimes in detail where these matters are themselves highly contested, but who they were and what they believed are not the only, or even the main, subjects here. Instead, the chapters are focused on the development and deployment of the rhetorical notion of ‘heresy’ throughout Christian history, examining how heretical ‘sects’ or ‘movements’ were constructed and tracing continuity and innovation across a variety of different contexts. The two most productive periods mentioned earlier – late antiquity and the mid to late medieval period – are particularly well represented, both in themselves and also for their influence on later centuries, including the repeated reuse of heretical labels and anti-heretical texts and practices which were created during them. The final ‘heresies’ to receive sustained attention here are those which were created just on the cusp of the Reformation – the ‘lollards’ and ‘Hussites’ – and are often interpreted (and celebrated) as part of that dramatic development in Christianity, as J. Patrick Hornbeck II and Pavlína Rychterová describe in their contributions. Nonetheless, the later history of heresy is recounted in several of the chapters, including Pete Biller’s on the Waldenses and John Tolan’s on Islam, demonstrating how ideas which emerged in the Middle Ages continued to play central roles in polemic and debate well into the early modern and modern periods.

Part I (‘Approaches and Evidence’) opens with John H. Arnold’s chapter on ‘Theories and Definitions of Heresy’, examining how both the notion of heresy itself and scholarly approaches to it have changed over time, thus providing a methodological underpinning for the rest of the volume and a route map to help students of this subject navigate their way through the various possible pitfalls. The other four chapters in this first section deal with different forms of written material that have played vital roles both in historical disputes and in modern reconstructions of these events, focusing on those which are most distinctive and offer the greatest challenges of interpretation for scholars. The first two of these – Richard Lim on ‘Dialogue Literature’ and Richard Flower on ‘Heresiological Catalogues’ – deal with genres which developed within a wider intellectual milieu in the classical world, providing further demonstration that Christians and their literature should not be viewed as clearly separated from the Roman world of which they were part. Both chapters focus on late antiquity as the most innovative period for these particular forms of writing but also examine some later developments and explore the ways in which they could be deployed in diverse contexts, both to defend prevailing ‘orthodoxy’ and sometimes to express dissent against it. Rebecca Rist, in a contribution on the topic of ‘Councils, Popes and Canon Law’, examines these interlinked institutional and legal methods for defining and defeating heresy. While some of these can also ultimately trace their origins back to antiquity, they became much more significant and developed in the later period covered by this volume, particularly from the eleventh and twelfth centuries onwards, including with the publication of Gratian’s Decretum. Rist explores a range of different forms of material in order to provide a nuanced narrative of how the Church responded to heresy in this period, including by creating new policies, processes and textual collections for this purpose. The final chapter in this section also looks at ‘Inquisition and Trial Records’, a form of literature which is distinctive to the later medieval and early modern period. In this contribution, Christine Caldwell Ames explores the wealth of surviving material, especially trial transcripts and inquisitorial manuals, and discusses the challenges involved in looking for the messy reality that lies behind their pages.

Part II (‘Case Studies’) examines thirteen different notable examples of the discourse of heresy within Christianity, spanning the volume’s entire chronological range. Just as this Companion is not a History of heresy, nor is it a Dictionary or Encyclopaedia: in a volume of this type it would be neither possible nor desirable to provide a complete catalogue of every person, group or idea that has ever been labelled thus, thereby imitating heresiologists such as Epiphanius of Salamis and Augustine of Hippo. Instead, the subjects for these case studies have been selected for their significance to the history of the concept of ‘heresy’ and its use, rather than merely representing the most prominent theological disputes that have taken place within Christianity: some, such as Pelagianism and the Cathars, were the subject of notable anti-heretical activities in their own day; some, such as Gnosticism, Manichaeism and Arianism, became common elements in the vocabulary of heresy, being repeatedly redeployed over the succeeding centuries to condemn other ‘sects’ by association; and some, such as the ‘Hussites’ and the presentation of Islam as a Christian heresy, illuminate different ways in which heretical discourse was enmeshed in wider contexts and conflicts. Some readers (and reviewers) may feel disappointed that a particular heresy has been chosen for inclusion while their own favourite one has been cruelly shunned, despite their assessment of its greater significance. It is hoped, however, not only that this volume achieves decent chronological and geographical coverage despite the practical limits imposed upon it but also, more importantly, that it explores a wealth of methodological and historiographical issues which are of relevance and use for the study of all the other heresies which do not receive sustained attention within it.

It should also be stressed that no attempt is being made here to pick a side in these historical disputes, to decide who ‘really deserves’ to be called orthodox or heretical. While some of the beliefs and practices described in this volume were widely condemned and attacked, leading to their eventual disappearance, many of these processes and arguments instead contributed to lasting divisions within Christianity, between churches now known by names such as Protestant, Catholic, Orthodox, Oriental Orthodox or Church of the East. Accounts of late antique and medieval heresy have frequently reproduced ‘Orthodox’, ‘Catholic’ or ‘Protestant’ narratives, especially, in the latter case, when discussing heresies which appeared close to the advent of the Reformation, as Chapter 17 discusses. While the study of the rhetoric of heresy necessarily focuses on the voices of those who employed such condemnatory language, the aim of this Companion is to explore the various actions, processes and texts involved in producing ‘heresies’ but certainly not to continue with that enterprise from any partisan or confessional position.

The case studies in Part II begin with Michael A. Williams’ exploration of ‘Gnosticism’, a highly contentious label traditionally applied to a range of ideas and ‘groups’ from early Christianity lumped together under this umbrella term. Williams explores how the category of ‘gnostic’ came into being, focusing on the rhetoric of Justin Martyr and Irenaeus, whose writings also played a major role in the creation of the notion of ‘heresy’ itself, as well as exploring some of the challenges involved in studying the theology of this period without reproducing heresiological categorisation. In contrast, there is no such concern about the existence of ‘Manichaeism’, as Jason BeDuhn explains in his chapter on this topic. Rather than being a phantom in the minds of ‘orthodox’ Christian authors, this was another form of Christianity, a religion with a clear ‘church’ structure and theology, engaging in debate with its opponents, including by employing its own accusations of ‘heresy’. Despite the widespread persecution of Manichaeans, the term lived on for many centuries as one of the archetypal heresies for other Christians, repeatedly deployed in arguments against contemporary opponents. The same use was also made of ‘Arianism’, which is examined in Robin Whelan’s chapter on this and other Christological disputes of the period. Whelan traces the theological and ecclesiastical convolutions of the fourth century, including the development of ‘Homoian’ theology and its place in a number of the ‘successor kingdoms’ to the western Roman empire, often being referred to as ‘Arian’ by its opponents despite differing significantly from Arius’ own beliefs. The same polemical construction of a category can also be seen with ‘Pelagianism’. Ali Bonner’s chapter on Pelagius examines the theological disputes often characterised as the ‘Pelagian controversy’ and the problems involved in reconstructing Pelagius’ own views. As Bonner argues, ‘Pelagianism’ was a category created during these disagreements to delegitimise certain beliefs as dangerous innovation, thereby constructing a heresiological label which, like so many others, could be reused centuries later in different contexts.

Theological arguments of the fifth century also take centre stage in J. Edward Walters’ chapter on ‘The Nestorian Controversy’, which explores the complicated disputes around and resulting from the councils of this period, especially Ephesus in 431 and Chalcedon in 451. As Walters demonstrates, the heresiological rhetoric formed during this period has had lasting impacts on Christian communities down to the present day, including the employment of terms such as ‘Nestorian’ and ‘Monophysite’ to describe beliefs and ecclesiastical structures which emerged from these controversies. John Tolan also considers the long legacy of late antique labelling in his piece on ‘Islam as a Christian Heresy’, starting with the eighth-century author John of Damascus, who positioned Islam as the hundredth and final entry in his catalogue of haireseis. Tolan then traces the use of this notion in a variety of contexts across Christian history, both in polemics against Islam itself and as part of disputes within Christianity, including between Catholics and Protestants in the early modern period. The eighth century also marked a major escalation in arguments about the status of images within Christianity, as Judith Herrin describes in her contribution on ‘Iconoclasm’. This chapter examines the origins of these debates and the shifting positions, together with mutual accusations of heresy, adopted by different Byzantine rulers until the final ‘triumph of orthodoxy’ in the ninth century. The impact of this dispute was, however, also felt elsewhere, most notably through the reception of documents about the controversy in the Frankish realm, and resurfaced later, especially during the Reformation. Contact and disagreements between ‘Latin’ and ‘Greek’ Christians are then the focus of the chapter on ‘Accusations of Heresy between East and West’ by Marie-Hélène Blanchet and Nikolaos Chrissis, exploring this phenomenon from the first disputes in the fifth century all the way to the fall of Constantinople in the fifteenth. Eschewing any simplistic division between ‘religious’ and ‘political’ motives in these events, Blanchet and Chrissis track the complexities of the rhetoric used by various parties and the significance of particular moments, including the ‘Schism of 1054’ and the sack of Constantinople in 1204, exploring when and how the language of ‘heresy’ was employed and, perhaps more importantly, when authors chose to hold back from it.

Andrew P. Roach’s chapter considers the challenges inherent in reconstructing the history, ideas and practices of the Bogomils, famously described in the influential tenth-century account of Kosmas Presbyter, who was particularly concerned with remedying the problems he saw within the Bulgarian Church of his day. By situating both Kosmas’ text and the origins of Bogomils firmly within the context of Bulgarian Christianity, Roach explores the evidence for the group’s beliefs, in particular the accusation of dualism, and then traces its later history, especially polemic and imperial action against Bogomilism in Byzantine society and the vexed question of its influence on ‘Cathars’ in western Europe. This ‘sect’ then takes centre stage in Jörg Feuchter’s contribution, which seeks to guide the reader through the significant, and sometimes heated, scholarly disagreements about whether the Cathars really existed as an organised group or were simply another heresiological fantasy dreamed up by churchmen and inquisitors. While acknowledging that he is ‘not a neutral observer’ in this debate, Feuchter explores the history of the different positions on the topic and the arguments for and against them, before concluding with a statement of his own position concerning the existence and identity of the ‘Cathars’.

Pete Biller’s chapter also focuses on another ‘heresy’ which looms large in inquisition records and manuals from the later Middle Ages: the ‘Waldenses’. In this lively contribution, Biller explores the difficulties involved in reconstructing the reality behind these textual accounts, as well as tracing the history of the interpretation of the Waldenses, moving through Reformation polemic all the way to current scholarship. This includes recent debates over the term ‘Waldensianism’ and its pluralised form ‘Waldensianisms’, shining a light on the different communities that existed across Europe and the relationships between them. Some similar issues are also raised by J. Patrick Hornbeck II’s chapter on ‘Wyclif, Lollards and Popular Heresy’, picking apart the terms ‘Wycliffite’ and ‘lollard’, both in so far as they relate to each other and because they run the risk of homogenising a range of different views and practices. Hornbeck instead examines what can be known about lollard beliefs, organisation and history, including the role of John Wyclif himself, and how these have been interpreted by later generations, including as part of teleological narratives regarding the Reformation in England. The volume then concludes with Pavlína Rychterová’s exploration of the events which were set in motion by the reception of Wyclif’s ideas and writings at the University of Prague from the end of the fourteenth century, especially within the circle led by Jan Hus. The debates created by this situation drew in representatives of both ecclesiastical and secular authority, as well as opening up existing fault lines between the two and ultimately leading to the ‘Hussite revolution’. Rychterová provides a guide to the many factors at play in the controversy during Hus’ own lifetime, the development of modern historiography, including the influence of nationalist movements, and the dispute over the use of the term ‘heresy’ to describe Hus or the Hussites.