In 1095, in a carefully crafted speech before prelates and nobles in Claremont, France, Pope Urban II called Europe to action: A Crusade to aid the Christian empire of Byzantium. Emissaries of the emperor in Constantinople had come to Urban to ask for aid against the advancing Muslim Turks, who were mistreating conquered Christians, desecrating shrines, and pressing on toward Constantinople. The response was sensational and spread immediately across Europe. Knights, clerics, and peasants all heeded the call and marched to the East—toward Byzantium, Antioch, and Jerusalem.

In July 1099, four years after Urban’s call to Crusade, Jerusalem fell to the Crusaders. It was a triumph marred by unspeakable violence. The Muslim and Jewish inhabitants of the city were slaughtered, almost to a man. The chronicler Fulcher of Chartres wrote of wading through ankle-deep blood. These horrors would haunt not only the Crusaders but Muslim-Christian relations for a thousand years.

Around this time, a less well-known, though no less significant, event took place.

Late in the eleventh century, after much reflection, the Muslim philosopher Abu Hamid al-Ghazali completed The Incoherence of the Philosophers. It may have been the most influential book in all of Islam after the Qur’an. Islam had initially encountered Greek thought with an open mind in what was known as Islam’s Golden Age. This period saw the great philosopher Avicenna reconcile Aristotle with Islamic revelation, as Aquinas would later do with Christianity. Ghazali rejected this synthesis of faith and reason, concluding that causation and free will were illusory, as God’s direct intervention was the source of each cause and each motion. Reason itself was but a human construct, its parameters insufficient to contain God’s will—will that could contradict itself in defiance of human comprehension.

Ghazali’s work was the epitaph of Islam’s encounter with Greek philosophy, of hellenized Islam, and of Sunni Islam’s experiment with faith and reason. As Ghazali’s movement to dehellenize—that is, to root out all rational analysis, all philosophy, all reason—gained ascendancy in the Muslim world, the interreligious, intellectual, and cultural engagement that had characterized the era of medieval philosophy drew to a close. It may well be argued that the Muslim world has been in decline since.

The twelfth-century Muslim philosopher Averroes attempted to refute Ghazali and to rehellenize Muslim scholarship and culture. He failed. Averroes was banished, his books were burned, and the teaching of philosophy prohibited—so complete was Ghazali’s triumph. With his banishment ended the last meaningful philosophical dialogue between the Muslim world and the West.

In 2006, a millennium after Urban’s call for a Crusade, Pope Benedict XVI gave a lecture at the University of Regensburg in Germany, to address the crisis of reason in the West. The influence enjoyed by the papacy had diminished significantly in the intervening thousand years; no longer would rulers stand in the snow to beg forgiveness. If not a “prisoner of the Vatican,” the pope now saw his ambit limited by a public culture that was increasingly secularized and hostile. The Vatican could scarcely rein in Catholic academics, let alone shape the ideas of greater academia. Philosophy had been declared dead in the West by materialist thinkers as it had been centuries before by the fundamentalist Ghazali. It was precisely the West’s break with reason—its dehellenization—on which Benedict focused his remarks.

The vital fusion of faith and reason—of Athens and Jerusalem—that had been part of Christianity since the early centuries had been divided by the Reformation and corollary movements, Benedict argued. To preface his argument, he quoted the words of another scholar under siege, the late Byzantine emperor Manuel Paleologus, who had engaged in a dialogue with a Muslim prince on the subject of God’s nature and man’s freedom. Benedict recalled the emperor’s contention that “violence is incompatible with the nature of God and the nature of the soul. ‘God,’ Paleologus said,

‘is not pleased by blood—and not acting reasonably is contrary to God’s nature. Faith is born of the soul, not the body. Whoever would lead someone to faith needs the ability to speak well and to reason properly, without violence and threats. . . . To convince a reasonable soul, one does not need a strong arm, or weapons of any kind, or any other means of threatening a person with death.’

Benedict continued:

The decisive statement in this argument against violent conversion is this: not to act in accordance with reason is contrary to God’s nature. The editor, Theodore Khoury, observes: For the emperor, as a Byzantine shaped by Greek philosophy, this statement is self-evident. But for Muslim teaching, God is absolutely transcendent. His will is not bound up with any of our categories, even that of rationality. Here Khoury quotes a work of the noted French Islamist R. Arnaldez, who points out that Ibn Hazm went so far as to state that God is not bound even by his own word, and that nothing would oblige him to reveal the truth to us. Were it God’s will, we would even have to practice idolatry.

As with Urban’s speech at Claremont, Benedict’s address gave way to violence, though unlike Urban, this was not what Benedict had hoped for. The speech was widely condemned in both the Muslim world and the West. Ironically, few of those who expressed outrage appear to have read it; indeed, few critics seemed to be aware that the speech was principally about the West—not the Muslim world.

What are the consequences of dehellenization? For the Muslim world, one consequence has been plain: Faith unmoored from reason has led to widespread violence in the name of that faith.

For the West, dehellenization has led to the rejection of all non-material categories of knowledge, of the metaphysical. Such ideas are not as innocuous or as irrelevant to our lives as they may appear. That man may know Reason, and through it the mind of the Creator of the cosmos; that this Creator writes the law into the very nature of man; that using violence as a means of conversion is contrary to the Divine will; that the freedom to choose faith is written into the nature of man by that God—these are powerful ideas with profound implications. Such ideas were a predicate to the dialogues of Muslim and Christian scholars of the medieval era. These ideas are presently rejected by both mainstream Sunni Islam and Western secularists, especially academics.

Ghazali’s campaign of dehellenization may be as obscure as the Crusades are infamous, but this medieval idea is perhaps more to blame for violence in the Muslim world than medieval knights. If the dehellenization thesis is correct, then the West’s secular approaches to end religiously based violence by means of war, democracy, foreign aid, or other policies are doomed to failure before they begin. If Benedict is correct, then philosophical reengagement is the true basis for peace—a peace that was lost not on a battlefield but centuries ago in the realm of medieval philosophy.

Andrew Doran served on the Executive Secretariat of the U.S. National Commission for UNESCO at the U.S. Department of State, where he has since worked as a consultant. His views are his own.