The study of heresy in the Middle Ages has largely focused on the rise of heresy and mechanisms devised to contain it in the Latin West. It is by now a well-accepted truism that the history of heresy must be understood through the lens of the history of orthodoxy. Heretical beliefs and practices became such primarily in the light of efforts to define and then protect orthodoxy, since they became apparent through the prosecution of doctrinal transgression. In Medieval Heresies: Christianity, Judaism, and Islam, Christine Caldwell Ames accepts this truism, but complicates it by knocking the Latin church, as the policing agent, from the center of the narrative, then refocusing attention on the phenomenon of heresy and its persecution across faiths and in multiple contexts.

Caldwell Ames adopts and adapts R. I. Moore's argument that the emergence of professionalized bureaucracies, efforts to regulate western orthodoxy, and the drive to expand the physical and ideological boundaries of western Christendom in the high Middle Ages fed a new and well-organized effort to identify and eradicate internal threats to the cohesion of Christian society—in other words, to produce official categories of outsiders. Starting with the premise that the twelfth and thirteenth centuries represented a period of significant concern about defining and protecting the terms of social, cultural, and religious belonging in Christendom, Caldwell Ames posits that similar movements crystalized in the Byzantine, Jewish, and Muslim contexts. Yet she takes issue with Moore's conclusion that this movement emerged from the newly centralized and politicized church. Indeed, as she shows, the organized effort to identify and eradicate heresy predated and outlived the heyday of centralized papal authority. Moreover, she argues that this process unfolded also beyond the religious and cultural geographical parameters of the Latin West. A focus on processes of identifying and punishing heresy across religious contexts reveals what Caldwell Ames calls a distinctly medieval form of heresy that found its strongest expression between the tenth and fifteenth centuries but that was also deeply rooted in historical understandings of and methods for discovering the true meaning of revealed texts. “What is distinctly ‘medieval’ about heresy in Judaism, Christianity, and Islam in these centuries,” Caldwell Ames argues, “is that it was a present danger, but also one inseparable from the past” (27).

Caldwell Ames's method is not purely comparative. The scope of the study does not permit the kind of lingering reflection a truly comparative work would demand. Rather, she structured the book as a chronological survey exploring the emergence and later persecution of particularly medieval heretical movements in a variety of social, cultural, and religious settings from the seventh through the fifteenth centuries. As such, each chapter sets clear chronological boundaries and defines a thematic or doctrinal common ground that Caldwell Ames uses to characterize and assess shifts taking place in the period under consideration. Chapter 1, which examines the period from 380 to 661 c.e., focuses on movements to establish structures of religious authority within eastern and western Christianity, Judaism, and early Islam. Chapter 2, dealing with 661–1031 c.e., moves on to address efforts to centralize or strengthen temporal governance. Hand in hand with concerns about social order amongst Christians, Muslims, and Jews, Caldwell Ames argues, came an increased sense that organized forms of social deviance represented a direct threat to society at large. The third chapter addresses the intellectual and social mechanisms deployed across religious contexts by religious authorities to define and preserve a norm of behavior and observance from 1031 to 1209 c.e.. The next chapter, covering 1209–1328 c.e., chronicles an alliance between temporal and religious authorities’ efforts to contain and control religious deviance. Finally, chapter 5 argues that from 1328 to 1510 c.e. newly emerging notions of nationhood also brought about an effort on the part of secular governments to identify heresy as a threat to the health of the nation.

Caldwell Ames structures her comparative study of medieval orthodoxy around a relatively traditional narrative of the trajectory of western Christian orthodoxy and the rise and fall of institutions designed to preserve it. Her choice to ground the study in a decidedly Christian framework reflects the author's area of expertise. Not surprisingly, her engagement with the Christian issues and texts demonstrates a breadth and depth of knowledge that she cannot replicate in the sections on Judaism, Islam, or Orthodox Christianity, in which she relies on secondary scholarship and translations. However, this strategy will serve as a useful point of entry for students who bring a relatively basic familiarity with medieval history but know little about the other communities Caldwell Ames treats in this expansive study. Caldwell Ames's argument that a distinctively medieval form of heresy crossed geographic, religious, and political boundaries adds an important layer and challenge to the scholarship on heresy (and orthodoxy) as the product of local historical, political, or social contingencies. What emerges from this approach is a fairly strong foundation for the case that activist efforts on the part of the western church had a far-reaching impact among Jews, Christians, Muslims, and eastern Christians across vast physical territories, even to the point of influencing similar conceptions of and responses to behaviors or theologies that were deviant—or were associated with deviance. Caldwell Ames brings sufficient evidence to suggest that mechanisms of authority developed within each of these faiths at roughly the same time. As such, this study provides a very useful reminder that the medieval world was far better connected and integrated culturally, intellectually, and theologically than we generally imagine.