The problem of heresy (from the Greek hairesis, meaning “choice”) was a dominant concern of the Catholic Church throughout the later Middle Ages. If we wish to understand the reason for this, we may profitably compare the medieval heretic to his secular analogue: the traitor.

Treason is to the modern state what heresy was to the medieval church. Indeed, the word “traitor” sprang from a religious controversy. Traditores (“handers-over”) were those Christian bishops who obeyed an edict of the Roman Emperor Diocletian (c. 303) to hand over copies of Scripture to the state so they could be destroyed. The traditores caused a major schism in North Africa, as theologians wrangled over whether traditores should be restored to communion and whether bishops who had been traditores could perform valid ordinations. The Donatist schismatics argued that traditores had lost all such privileges and that ordinations by such handers-over were invalid; whereas Catholics such as Augustine took the opposite view, maintaining that the spiritual powers of bishops lay in their office and remained valid regardless of the sins of particular bishops. (Augustine believed that the exclusionary policy of the Donatists would prevent the Catholic Church from becoming truly universal—an organization that included both sinners and saints—and would doom it instead to the permanent status of a relatively small group consisting only of the pure.)

Clearly, treason implies betrayal, but one must belong to an organization before one can betray it. This is what distinguishes the heretic from the infidel. The infidel is an outsider, a nonmember who is incapable of betraying a church to which he never belonged. Likewise, a foreigner cannot commit treason against any government but his own.

During the Middle Ages the individual was inducted into the church through baptism. Since this usually occurred during infancy, it could scarcely be called voluntary. Nevertheless, the godparents made promises on behalf of the child that obligated him or her for life. Thereafter the Christian, bound to the church legally and irrevocably through baptism, was required to defer to official doctrine, as ultimately determined by the pope. In the words of the medieval historian Walter Ullmann (The Individual and Society in the Middle Ages, Johns Hopkins, 1966, p. 37), heresy was the sin committed by a Christian who “showed intellectual arrogance by preferring his own opinions to those who were specially qualified to pronounce upon matters of faith.” Heresy was more than a sin, however; it was also a crime. The heretic committed high treason against the political authority of the church and endangered the theocratic foundation of government. Orthodoxy (“right thinking”) was the ideological bedrock of social and political order, and the heretic threatened to undercut this foundation at its root. The parallels between heresy and treason are therefore more than superficial. Heresy was itself punishable as treason because it subverted the authority of the church from within.

It should be noted that the notion of orthodoxy per se (i.e. shorn of its religious connotations) is unobjectionable, even to atheists and other freethinkers. An organization must establish an identity by which to distinguish between members and nonmembers. If an organization is ideological in nature, then its identity will be defined by a credo—a set of beliefs or principles that determines the conditions of membership. The ability of that credo to withstand philosophic scrutiny is irrelevant here. Whether rational or irrational, a credo constitutes the “orthodoxy,” the intellectual foundation, of an ideological organization. An organization without an orthodoxy is an organization without ideas.

Thus understood, an orthodox credo is a feature of all ideological institutions. Because philosophy normally lacks institutional affiliations, it (ideally) does not have credos, orthodoxies, and heretics. This is the nature of the beast called philosophy, but it does not render philosophers inherently more noble, honest, or undogmatic than their theological cousins. Potential problems for theologians arise from the fact that most are at once church members and philosophers, and those roles may conflict. If a philosopher joins an ideological organization and commits to its credo, then he too may experience the tension between orthodoxy and heresy—between loyalty to an organization (or movement) and loyalty to the voice of reason. I see no evidence to suggest that philosophers, if confronted with this decision, will necessarily choose better than theologians.

When viewed in an institutional setting, the ideas of orthodoxy and heresy, broadly conceived, are neither irrational nor dangerous—so long, that is, as membership in an institution is voluntary. Dissenters are free to quit an organization or never to join at all. This was the case with early Christianity before it became the state religion in fourth-century Rome. Christianity was more than a theology; it was also an organization, and as such it needed to forge an orthodoxy, an intellectual identity, a set of beliefs that unified its members. A heretic in those circumstances might be expelled or banned from a particular organization, but he would remain free to go his own way.

Only after Christians acquired the reins of state power and substituted force for persuasion did its notions of orthodoxy and heresy become dangerous. Through the instrumentality of government what had been orthodoxy for a single religious movement became orthodoxy for society as a whole. As the distinguished medievalist R.W. Southern observed in Western Society and the Church in the Middle Ages (Penguin Books, 1970, p. 17), “the church was a compulsory society in precisely the same way as the modern state is a compulsory society.” As the good of the church became synonymous with the good of society, heretics became more than wayward members of an organization—they became threats to the foundations of social order. This transformation was caused not by the notion of orthodoxy per se but by the politicizing of orthodoxy.

Heretics were ferociously persecuted by Catholic authorities at various times during the Middle Ages. One of the most atrocious examples occurred in the early thirteenth century in southern France, during what is usually called the Albigensian crusade. Thousands of heretical “Cathars” were slaughtered indiscriminately—men, women, and children alike—and entire towns were devastated, even though the Cathars kept to themselves and threatened nobody. Upon learning of those wholesale massacres, a delighted Pope Innocent III gave thanks for a double blessing: Wicked heretics were being killed, and their killers were that much closer to attaining salvation. As this supposedly great pope put it:

God hath mercifully purged His people’s land and the pest of heretical wickedness…is being deadened and driven away….Wherefore we give praise and thanks to God almighty, because in one and the same cause of His mercy, He hath deigned to work two works of justice, by bringing upon these faithless folks their merited destruction, in such a fashion that as many as possible of the faithful should gain their well-earned reward by the “extermination” of those folk. [Quoted in G.G. Coulton, Inquisition and Liberty, Beacon Press, 1959, pp. 103-4.]

Historians have long been aware that medieval religious persecution—which became centralized under papal control (known as the Holy Office of the Inquisition) not long after the Albigensian crusade—often had political undercurrents. But some of those historians have offered a defense (of sorts) of the persecutors. We are told that rulers and church authorities were generally more lenient and open-minded than the people they ruled. As R.W. Southern (p. 17) put it: “on the whole the holders of ecclesiastic authority were less prone to violence, even against unbelievers, than the people whom they ruled.” According to another historian, heretics “aroused intense feelings of fear and hatred among the mass of the people because they dissociated themselves completely from all the values on which society was based.”

This conventional interpretation was challenged by R.I. Moore in a brilliant and provocative book, The Formation of a Persecuting Society: Power and Deviance in Western Europe (Basil Blackwell, 1987; 2^nd ed. 2007). Moore questioned whether the masses feared and despised heretics as much as some historians have claimed; on the contrary, heresies often attracted substantial segments of the population. Generally speaking, according to Moore, it is not true that “medieval man” feared and resented any deviation from the Catholic faith; rather, “the reason why preachers of heresy were denounced, pursued and extinguished by whatever means availed was precisely the fear that they would undermine the faith of the simplices, and with it the social order.” After examining various cases in which heretics appear to have been executed by public demand, Moore concluded that “on inspection their numbers shrink rapidly.”

According to Moore, we possess “no true evidence of general popular antipathy to heresy as such.” Thus medieval persecutors were not “agents of society at large, at least if our conception of society is one which includes the great majority of its members.” Persecutors had their own agenda; heretics were a political threat because they declared “their own independence of the structures of established power. They looked for their authority to those who heard them.” Emerging bureaucratic regimes stigmatized and persecuted heretics—along with Jews, homosexuals, witches, and even lepers—as a means to consolidate and justify their own power.