Intellectual history has long since fallen from favor, having been replaced within the academy by the new social history, cultural history, gendered history, and what have you. But there was a time when intellectual history reigned supreme. Its practitioners took for their study the most important and expansive of subjects—like the ideas of freedom, equality, or democracy. It is true that some of these “history of ideas” studies could seem, in the wrong hands, rather canned. But in the right hands, they were history at its very best, and we are fortunate that it is still practiced at its highest level by a master like Perez Zagorin.

An emeritus professor of history at the University of Rochester, Zagorin has authored well-received books on Francis Bacon, Thucydides, and Milton, as well as many other rich and thoughtful historical studies. More recently, Zagorin completed How the Idea of Religious Toleration Came to the West. Appearing just a few years after the September 11 attacks—surely one of the most extreme cases of religious intolerance in modern times—Zagorin’s scholarly study gained an urgency it might otherwise not have had.

In writing an intellectual history of toleration, Zagorin is covering well-trodden ground. We already have W.K. Jordan’s The Development of Religious Toleration in England, Joseph Lecler’s Toleration and the Reformation, and Henry Kamen’s The Rise of Toleration, not to mention many more specialized studies of such thinkers as Spinoza, Locke, Bayle, and others who were central in the story of toleration’s rise and spread. But Zagorin’s book is undoubtedly the best general history to date. Where earlier studies tended to be overly detailed or oversimplified, Zagorin has found just the right balance between careful exegesis of key toleration texts and useful explanation of the historical context in which the debates took place.

A complete history of toleration must begin with its pre-history. For as Zagorin points out, the principle of toleration came to the West rather late, having to supplant what Lord Acton called “a system of Persecution”—or what James Madison called “that diabolical Hell-conceived principle of persecution.” Hell-conceived, yes, but as Zagorin makes clear, a lot of great Christian thinkers defended persecution, deeming it perfectly in accord with—even demanded by—the words of Jesus himself.

No less a Christian sage than Augustine offered elaborate justifications for the persecution of heretics—including killing them if necessary. It was in his theological struggles with three heresies—Manichaeanism, Pelagianism, and Donatism—that Augustine first came around to defending coercion in religion. By his own account, he was originally opposed to coercing religious belief, but when he discovered that it seemed to work, that Donatists could be successfully “converted to Catholic unity by the fear of imperial laws,” he had a change of mind. If physical pain could convert the wayward to the true faith, then one was only administering them a great mercy. Better for them to suffer a few lashings in this world than suffer eternally in hell. Thus did Augustine come to endorse what Zagorin aptly calls a “pedagogy of fear to effect a change of heart.”

Augustine also found justification for a coercive policy in Jesus’ “parable of the tares” in the Gospel of Matthew as well as in the “parable of the feast” in the Gospel of Luke. This latter parable, in particular, would become central to subsequent debates over toleration. The Gospel tells of a man who prepared a great feast, and then told his servants to “go out in the highways and hedges, and compel them to come in, that my house may be filled.” As Zagorin explains, Augustine took this as a clear scriptural mandate to compel people “to do right.”

From this point onward persecution became an accepted Christian principle and practice, one that would culminate in the Inquisition and the wars of religion between Protestants and Catholics. The penalty for heresy was death (frequently by fire), and in one especially bloody period thousands of heretics received this legally sanctioned penalty. Even that humane philosopher Thomas Aquinas gave his blessing to such practices, declaring that heretics “deserve not only to be separated from the Church by excommunication, but also to be shut off from the world by death.”

As Zagorin ruefully comments, a distinctive and powerful “Christian theory of persecution” emerged—concocted not by criminals or sadists but by some of Christianity’s greatest minds and most pious adherents. “Because this theory was embraced by men of high moral character,” he explains, “it is possible to describe the religious persecution of earlier centuries as persecution with a good conscience.” This would cause difficult problems for any possible reformation. If persecution were ever to be overcome, a genuine transvaluation of values would have to be effected.

In the Renaissance period, several notable thinkers took the first tentative steps away from the principle of persecution. Philosophers like Erasmus and Sir Thomas More were appalled by the cruelty of the pious, though as Zagorin points out, “religious concord,” not toleration, was what they favored. Erasmus was a virulent anti-Semite, and More wrote in favor of the burning of heretics. As for the Reformation, it only intensified the levels of religious hostility and persecution in Europe. Calvin and Luther were no more tolerant than Augustine or Aquinas before them. Europe’s wars of religion brought the continent to such a state of exhaustion that simple political expediency forced upon its combatants a policy of “live and let live.” The Edict of Nantes, passed in 1598, ushered in the first official toleration. But it was not toleration in the name of high principle: the two sides were simply too exhausted to keep killing each other.

In this period of relative quietude, Zagorin says, the first genuine advocate of toleration emerged in Sebastian Castellio, a sometime student and associate of Calvin. Castellio was prompted by the tragic case of Michael Servetus, a highly heterodox man of letters, biblical scholar, and scientist who was tried for heresy by the Catholic Church. Servetus pleaded for death by the sword, but to no avail, and was burned alive in 1553.

It was a grisly end for a great man, and Castellio was one of those who was profoundly affected by Servetus’ ordeal. In his pamphlets, he favored a comprehensive toleration among Christians. His arguments, Zagorin explains, proceeded along “very largely Christian grounds,” but Castellio went beyond Erasmus’ limited notion of a Christian concord, arguing that toleration itself was a virtue.

Zagorin next turns to the toleration debates in the Netherlands and England. Here we meet toleration advocates like Dirck Volckertszoon Coornhert, Hugo Grotius, Roger Williams, and John Milton. In Zagorin’s account, these thinkers were not trailblazers but heirs to Castellio who made the Christian case for toleration among Christians. “In advocating a policy of peace and tolerance toward religious differences,” Zagorin writes, “their supreme concern was the welfare of religion itself. They acted from the primary conviction that persecution was contrary to the mind of Christ and a terrible evil which did great harm to Christianity.”

These religious arguments were eventually superseded by what Zagorin calls the “final stage” in the evolution of the idea of toleration, the emergence with John Locke and Pierre Bayle of a genuinely universal and essentially secular toleration. Unlike their predecessors, Locke and Bayle were not “Christian thinkers” in the strict sense, but philosopher-trailblazers who argued in favor of a general toleration on the broadest possible grounds. Locke, for instance, states plainly that there is no such thing as a “Christian commonwealth” and argues for the toleration of Jews, Muslims, and even pagans (only atheists don’t make the cut).

Locke’s case for toleration proceeded along several lines. Taking issue with Augustine’s “pedagogy of fear,” Locke insisted, to use his own words, that “such is the nature of the Understanding, that it cannot be compell’d to the belief of any thing by outward force.” One might bludgeon another into conformity, but conformity is not the equivalent of sincerity (the only sort of belief acceptable to God, in Locke’s view).

Locke made an additional argument: Could it really be the case, as Augustine had claimed, that religious persecutors were acting in charity? Quite to the contrary, said Locke: “It will be very difficult to persuade men of Sense, that he, who with dry Eyes, and satisfaction of mind, can deliver his Brother unto the Executioner, to be burnt alive, does sincerely and heartily concern himself to save that Brother from the Flames of Hell in the World to come.”

Finally, Locke’s case rested on the claim that “everyone is Orthodox to himself.” In other words: Who is really to say what is orthodox or heterodox in matters of religious belief? From Locke’s view, it is simply beyond human reason to know which ways of worship and which beliefs are most pleasing to God. This was Locke’s skeptical challenge to claims of religious certitude, but Locke was no relativist. For at the very heart of his argument was his audacious claim that toleration is “the chief Characteristical Mark of the True Church,” and he would go so far as to insist that all “Ecclesiastical men” preach from their pulpits the “duty” of toleration. Indeed, he argued that those who reject the principle of toleration have no right to toleration.

Some have argued that by formulating a more relativistic case for toleration, Bayle moved beyond Locke. And certainly, Bayle did advance the concept of toleration by arguing that morality is completely independent of religion, and atheists are in fact more virtuous than Christians. Bayle also argued—perhaps more openly than did Locke—that the only way to interpret the Bible is by the “natural light” of human reason. But with both Locke and Bayle we have entered a new era in the toleration debate—one that favors on broadly universalist grounds a toleration among all men, whatever their religious beliefs.

Zagorin’s intellectual history ends here. In a concluding chapter, he briefly mentions those who followed in the footsteps of Locke and Bayle, including Kant, Montesquieu, Voltaire, Rousseau, and Jefferson and Madison. He also briefly reviews how toleration came to be enshrined in the 1946 United Nations Charter and in the 1965 Second Vatican Council. Zagorin’s point seems to be that, intellectually speaking, all the hard work and heavy lifting had already been done. Generally speaking, he views the history of toleration as progressive: Over the course of a long and sometimes tortuous path—he makes mention of the “hideous reverse” of the Holocaust—toleration came to define the West. He ends his book by expressing his hope that “despite the many adverse signs at present, we shall in time, and with the help of Western example,” witness the spread of the right of toleration to places like the Islamic world where it does not exist today.

Yet our present situation may be more dire than Zagorin lets on. Since his story essentially ends in 1700, he overlooks the troubling fact that the “Western example” is not what it once was. A more complete history would have considered not only the rise of toleration but also its fall over the last half century, and the implications of this decline for the West today.

For instance, many intellectuals today believe that the old principle of toleration cannot handle the new questions of race, ethnicity, culture, gender, and sexual identity. Not toleration but “recognition of difference” is what they seek in their quest for a truly “multi-cultural” society. To the extent that they think at all about toleration, they tend to be critical of its supposedly niggardly terms, and to question whether it is sufficiently inclusive of the new modes of “difference.” They complain of toleration’s narrowness and of its built-in Western biases. Having been freed of theological shackles, they now seek liberation from moral ones as well.

The philosopher Leo Strauss warned early on of this tendency in liberalism, noting that at its deepest levels the liberal conscience is at war with itself. The good liberal both honors toleration and celebrates diversity. These seem to be compatible aspirations, and up to a point they are, but eventually diversity presses against the limits of toleration, and then one or the other must go. Toleration, as conceived by Locke, requires a complete rejection of the persecuting spirit. But for good liberals who pride themselves on their openness even this seemingly obvious restriction leads to some embarrassment. And eventually, toleration’s single commandment (No toleration for the intolerant!) gives way to more forgiving standards: Let a thousand flowers bloom! Let the Nazis march in Skokie! Let the natives practice clitoridectomy! Let the widows be burned on the funeral pyre! Let London’s Islamic preachers of hate have their say!

Today, the principle of toleration is threatened on two fronts. On the one hand, it is under assault by post-modern intellectuals who find it just too constraining to satisfy their multi-cultural tastes or their desire for unfettered diversity. They would expand toleration to even the most intolerant. On the other hand, toleration is under attack, literally, by the pre-modern principle of persecution, as it has come to be embodied in radical Islam. Many in the West have all but ceased to believe in toleration as originally conceived, just at a time when it is once again under assault from its old enemy. It does not seem too much of an exaggeration to say that the future of the West will be determined at this theological-political juncture.

Whether we muster the courage to defend ourselves over the long haul will depend in part on whether we think toleration a thing worth defending. Can we any longer explain why toleration is superior to intolerance? Do we still believe that the best society is the tolerant one? And will we insist, as Locke did, that all the clergy (imams not excluded) preach the duty of toleration? By reminding us of toleration’s true foundations as well as of some of its necessary limits, Zagorin has written a timely book and a book for our times. Perhaps it will even contribute to reviving a Western example truly worthy of imitation.

Adam Wolfson is Consulting Editor of Commentary and a Senior Fellow at the Ethics and Public Policy Center.