Originally published at the Huffington Post.

The startling success of Islamist extremists in seizing major cities in Iraq and threatening the government in Baghdad has many causes, but none more important than this: the failure of modern Islam to reconcile its deepest beliefs with the principle of religious freedom. Much of the violence that has engulfed the Muslim world today — whether it involves Sunnis against Shiites, or governments against insurgencies — has its roots in this failure.

The West has an intimate knowledge of the problem: its pogroms, crusades, and inquisitions bear witness to it. Europe learned almost too late that no state or society could rest secure when it denied this universal human right — liberty of conscience — to any of its citizens. Like European Christianity on the eve of the Enlightenment, Islam must find a justification for pluralism and toleration, or condemn itself to an endless winter of human suffering.

Despite an official end to Europe’s wars of religion with the Treaty of Westphalia in 1648, violence motivated by religious belief continued to undermine political regimes in the seventeenth century. Whether in Protestant or Catholic countries, state-sponsored persecution was the norm: religious minorities across Europe faced discrimination, imprisonment, and even execution. Entire populations lived in the shadows as second-class citizens.

It was the problem of sectarian strife that absorbed the mind of English philosopher John Locke in the winter of 1685. With the ascension of a Catholic king on the throne, Protestant England was on the verge of another civil war. Living as a political exile in the Netherlands, Locke encountered religious minorities fleeing violent persecution in Catholic France. The atrocities committed in the name of the gospel appalled him.

The result was Locke’s “A Letter Concerning Toleration,” arguably the most influential defense of religious liberty in the West. Like no thinker before him, Locke sought to reform the Christian church as a prelude to a political revolution that would lay the foundation for liberal democracy. “It is not the diversity of opinions which cannot be avoided,” he wrote, “but the refusal of toleration to those that are of different opinions, which might have been granted, that has produced all the bustles and wars that have been in the Christian world, upon account of religion.”

Locke chided believers for imagining that authentic faith could be produced by threats and coercion. “All the life and power of true religion consists in the inward persuasion of the mind,” he wrote, “and faith is not faith without believing.” Compulsion in religion created hypocrites, he argued, not worshippers.

Hence Locke’s doctrine of the separation of Church and State, which profoundly influenced the American Founders. The alliance of these institutions invited corruption and conspiracies. Instead, Locke argued, the State must be indifferent to religious orthodoxy. Its job was to protect the right of all individuals to obey their conscience and join the faith community of their choice. By upholding this “law of equity,” the State would eliminate a major cause of civic unrest. For their part, churches would contribute to political and social stability “if the pulpits everywhere sounded with this doctrine of peace and toleration.”

Locke wanted to extend religious freedom not only to all Christian sects, but to religious believers of all kinds. In the Lockean state, all would enjoy equal justice under the law. “If we may openly speak the truth, and as becomes one man to another, neither Pagan, nor Mahometan, nor Jew, ought to be excluded from the civil rights of the commonwealth, because of his religion.” Given that anti-Muslim hatreds were at a fever pitch after the Ottoman Turkish assault on Vienna in 1683, Locke’s “Letter” embodied an astonishing civility and egalitarianism.

Now consider the barbarism that has descended upon the Middle East. In 2011, Bashar al-Assad, an Alawite, denied greater political freedoms to Syria’s Sunni majority, setting off a civil war and a massive humanitarian disaster. When Mohamed Morsi, a leader in the Muslim Brotherhood, was elected president of Egypt in 2012, he promised to be “a president for all Egyptians.” But his words — and his rule — evaporated into chaos, thuggery, and executions.

Nuri al-Maliki, a Shia Muslim, made similar promises when U.S. troops were still in Iraq. Instead he cracked down on Sunni political opponents and is paying the price in the sectarian violence that threatens the survival of Iraq. The ambitions of the Islamic State in Iraq and Syria are darker and more extreme than those of Mr. al-Maliki. But they are the logical result of a cultural norm that compels religious obedience and criminalizes dissent — a norm held throughout the Arab and Muslim world.

The question facing U.S. policymakers is inescapable: can Islam make its peace with religious pluralism?

For John Locke, the example of Jesus was the trump card. Locke appealed to “the Captain of our salvation” and “the perfect example of the Prince of Peace,” who showed grace to sinners and won over souls with compassion, not violence. “If the Gospel and the apostle may be credited, no man may be a Christian without charity, and without that faith which works, not by force, but by love.”

Muslim leaders must decide whether the life and teachings of Muhammad offer a basis for toleration. They must articulate a religious rationale for peaceful pluralism. If the example of the West is any guide, there is no other way out of the deadly maelstrom unfolding before our eyes.

Joseph Loconte is an associate professor of history at The King’s College in New York City and author of God, Locke, and Liberty: The Struggle for Religious Freedom in the West (Lexington Press, 2014).