As the Catholic Church went about exterminating heretics in the thirteenth century, major intellectuals rallied to her defense. Thomas Aquinas (1225-74)—a Dominican whose order had been specifically charged by the pope to root out heresy—discussed heresy in detail in his voluminous Summa Theologica, and his views profoundly influenced church policy. (In 1542, when Pope Paul III reconstituted the Holy Office of the Inquisition in Rome, he relied heavily on the arguments of Aquinas.)

Aquinas, like Catholic theologians before him, distinguished between unbelief and heresy. Pure unbelievers are those who have never accepted the Christian religion, such as Jews and Muslims. Those unbelievers should be punished if they blaspheme or commit other sins, but they should not be forced to embrace Christianity.  But heretics are a different matter, because heresy is a special kind of unbelief “pertaining to those who profess the Christian faith, but corrupt its dogmas.”

Unlike pure unbelievers, heretics should be compelled to accept the correct doctrines. Heresy involves more than error; it is obstinate error. Simple error may be innocent, but if the heretic persists in his error after being corrected by church authorities, then his error becomes willful—the result of egoistic pride. And when this happens the heretic should be punished. Why? Because heretics, having pledged themselves to God via the Catholic Church, “should be submitted even to bodily compulsion, that they may fulfill what they have promised, and hold what they at one time received.”

The heretic betrays his pledge to God and thereby corrupts the very religion he professes to accept. This affront to God, explained Aquinas, deserves the death penalty.

Heretics deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Therefore, if forgers of money and other evil-doers are condemned to death at once by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated, but even put to death.

Although heretics deserve death for the first offense, the church, merciful as always, should not inflict this penalty until the second relapse into heresy. This reasoned barbarism of Aquinas surpassed even that of Augustine, who had favored mere fines and banishment as penalties for heresy.

The Catholic Church regarded sixteenth-century Protestants as heretics worthy of death, and many of them paid that price. It is hardly surprising, therefore, that leading Protestant Reformers defended some measure of toleration, at least for themselves. Most of those same leaders, however, also defended the persecution of Catholics and even heretics among their fellow Protestants. When accused of inconsistency, those Janus-faced Protestants typically replied in the same manner as did John Calvin, namely: The right to persecute error does not entail the right to persecute truth. And since the Protestant leaders were absolutely certain that their doctrines, and their doctrines alone, were right, it followed that they had a right to the freedom necessary to preach the truth, as well as the right to suppress religious error by violent means, if necessary. 

In 1523, shortly after Martin Luther had split from the Catholic Church (and while his life was still in danger), he published a passionate defense of toleration. Luther cited “the well-known saying, found also in Augustine, ‘No one can or ought to be constrained to believe.’” He then summarized this time-honored argument: “it is useless and impossible to command or compel any one by force to believe one thing or another. It must be taken hold of in a different way; force cannot accomplish it.” Luther thus proclaimed the inability of persecution to accomplish its intended goal: “Heresy can never be prevented by force.” If God’s word (the Bible) and persuasion do not correct heretics, then “the end…will remain unaccomplished through secular power, though it fill the world with blood.”

Luther conceded that persecution can compel “by word and deed,” but such outward conformity, when it does not flow from inner conviction, is sheer hypocrisy devoid of religious significance. Persecutors “cannot constrain [the heart], though they wear themselves out trying. For the proverb is true, Thoughts are free.” Salvation is an unmerited gift freely bestowed by God “to which no one can be forced. Nay, it is a divine work, done in the Spirit, certainly not a matter which outward authority should compel or create.” (The preceding quotations are from Luther’s 1523 work on Secular Authority, in Martin Luther: Selections from His Writings, ed. John Dillenberger, Anchor Books, 1961.)

If Luther had written nothing else on toleration, he might have received accolades from future generations, believers and unbelievers alike. But as sympathetic biographers might say, Luther was a complex man, an unsystematic thinker who advocated different policies at different times. Or as unsympathetic biographers might say, an older Luther—ensconced in power, his life and future secure—betrayed his principles and turned from persecuted to persecutor.

Whatever the correct explanation may be, there is no doubt that Luther, after aligning himself with sympathetic German princes (many of whom were more interested in power and looting Catholic property than in religious doctrine), became a fierce advocate of persecution. We see this in Luther’s attitude toward the Anabaptists, or “rebaptizers” (a misleading label foisted upon them by their enemies)—those Protestants who believed in adult baptism and mocked infant baptism as “the popish bath.” Anabaptists were savagely persecuted by both Catholics and Protestants, and many thousands were cruelly tortured and slain. Protestants were especially fond of “baptizing” Anabaptist heretics by drowning them in rivers.

Luther initially defended freedom for Anabaptists (except the small minority who resorted to violence), but he later called for the extermination of all heretics, along with Catholics who did not acknowledge themselves enemies of Christ and the Emperor.

Heretics are not to be disputed with, but to be condemned unheard, and whilst they perish by fire, the faithful ought to pursue the evil to its source, and bathe their hands in the blood of the Catholic bishops, and of the Pope, who is a devil in disguise.

Jews also felt the sting of the hateful intolerance of the older Luther.

Burn the synagogues; take away their books, including the Bible. They should be compelled to work, denied food and shelter, preferably banished. If they mention the name of God, report them to the magistrate or throw Saudreck [pig dung] on them. Moses said that idolaters should not be tolerated. If he were here he would be the first to burn their synagogues.

It should be noted that even in later life Luther sometimes defended what he called “freedom of conscience” (a phrase he may have coined). But by this he merely meant the freedom to believe as one likes, according to one’s own judgment. This included heretical beliefs, provided one kept such beliefs to oneself. Once the heretic talked about his beliefs, however, or attempted to communicate them to others in any manner, or engaged in heretical rituals, then he should fall prey to the force of Christian law. In short, for the older Luther “freedom of conscience” signified the right to believe as you like, so long as you remain silent about any heretical beliefs you may have. Freedom of conscience did not include freedom of speech or worship, for those practices might easily infect others and lead them into heresy as well.

If Martin Luther provided erratic support for toleration, another great Reformer, the Frenchman John Calvin, provided none at all. As an eminent Christian biographer of Calvin once quipped, if someone claims to find a single word in favor of toleration in Calvin’s voluminous writings, it must be a misprint. Although Calvin rejected Catholicism, he embraced the Catholic policy of persecution: “Because the Papists persecute the truth, should we on that account refrain from repressing error?”

As a believer in predestination, Calvin understood that persecution is powerless to save souls, for the elect had been selected by God prior to creation. But Calvin insisted that heresy and blasphemy (the two concepts were sometimes used interchangeably by Protestant champions of persecution) insult the supreme majesty of God and should be punished for that reason. The punishment of heretics and blasphemers vindicates the honor of God.

Calvin was the hardest among hard-liners. Under no circumstances should secular authorities tolerate heretics and blasphemers. Indeed, “Those who would spare heretics and blasphemers are themselves blasphemers.” The “implacable severity” of the death penalty is mandatory because “devotion to God’s honor should be preferred to all human concerns.” And in pursuit of this end, we “should expunge from memory our mutual humanity” as we exterminate heretics and blasphemers with righteous zeal.

Calvin proved as good as his word. In 1553 he engineered the gruesome execution of the Spaniard Michael Servetus, a brilliant philosopher, scientist, and physician who is credited with discovering the pulmonary circulation of blood. Servetus was adjudged a criminal for two primary reasons, each punishable by death in Roman Law: He rejected the Trinity as a needless, incomprehensible doctrine, and he denied the efficacy of infant baptism. (Having repudiated the canon law of the Catholic Church, Protestants typically appealed instead to the ancient Code of Justinian for legal justification of their judicial murders.)

While hiding in France under an assumed name, Servetus sent a manuscript copy of his last book (The Restoration of Christianity) to Calvin in Geneva. Servetus apparently hoped the great Reformer would view the book sympathetically—a naïve and fatal miscalculation. Calvin never returned the manuscript; he had a better use for it. He gave four incriminating pages to a colleague, Guillaume Trie, who forwarded them to a Catholic correspondent in France. Trie also revealed the true identity of “Villanovanus,” the pseudonym used by Servetus. The Protestant Trie chided French Catholics for tolerating this heretic in their midst.

You suffer a heretic, who well deserves to be burned wherever he may be. I have in mind a man who will be condemned by the Papists as much by us or ought to be….Where I’d like to know is the zeal which you pretend? Where is the police of this fine hierarchy of which you so boast?

The tactic of Calvin and Trie almost worked. Servetus was arrested and convicted of heresy by Catholic authorities, but before he could be roasted over a Catholic fire he managed to escape from prison and head for Italy. Unfortunately, for reasons known only to himself, Servetus chose a route that took him through Calvin’s Geneva, where he was quickly recognized and, in an indictment drawn up by Calvin, charged with thirty-nine counts of heresy and blasphemy. Servetus was sentenced to death for attempting “to infect the world with [his] stinking heretical poison.”

On October 27, 1553, Servetus was bound to a stake with a rope twisted around his neck, his book tied to an arm, and a Protestant fire—fueled by green wood to make it burn slowly—stirring below. It took thirty minutes for Servetus to die, but by all accounts he suffered the ordeal bravely. Defenders of Calvin like to point out that he favored the more merciful execution of beheading rather than burning, but was overruled by Genevan officials. If that qualifies as a point in Calvin’s favor, so be it. In my judgment, among major thinkers the western world has rarely if ever seen a more intolerant, arrogant, and sadistic mentality than the mind of John Calvin.