Perhaps the most important event in Western history occurred on the road to Damascus, when Saul of Tarsus fell to the ground and converted from a persecutor of Christians to a believer himself.

His writings as the Apostle Paul make him arguably, as Larry Siedentop writes in his new intellectual history “Inventing the Individual,” the greatest revolutionary the world has ever known. Paul upset the assumptions of the ancient world and created the moral basis for modernity as we know it.

In his revelatory book, Siedentop explains how Christianity undergirded the development of Western liberalism, albeit slowly, with many detours, and sometimes inadvertently. It cleared the space for individual rights and ultimately for secularism itself, such that nearly everyone today — left and right, believers and militant atheists — stands on ground created by Christian moral premises.

Rather than the story of Christianity being a tale of unrelieved darkness and repression, as the hostile version has it, running straight from the Crusades and Inquisition to the contemporary low point of Indiana’s Religious Freedom Restoration Act, it created a world uniquely suited to human dignity and flourishing.

Siedentop begins his story with the ancients. The Greeks and Romans of prehistory weren’t secular; the family was a religious cult run by the paterfamilias and suffused with ritual and assumptions of social inequality. Even if you are pro-family, you can agree that ancestor worship … takes it a little far.

The key distinction at this time, Siedentop points out, wasn’t between public and private spheres, but between public and domestic spheres, the latter characterized by the family with its rigidly defined hierarchical roles. There was no room for the individual with his or her own rights.

“The most distinctive thing about Greek and Roman antiquity,” Siedentop writes, “is what might be called ‘moral enclosure,’ in which the limits of personal identity were established by the limits of physical association and inherited, unequal social roles.”

Christianity set in motion something completely different. It emphasized the moral equality of all people, and it made individual conscience a central concern.

“For Paul,” Siedentop writes, “the love of God revealed in the Christ imposes opportunities and obligations on the individual as such, that is, on conscience.” The source of authority had been reversed: “Increasingly it was to be found ‘below,’ in human agency and conscience, rather than ‘above’ in coercive eternal ideas.”

This was a radical development that contained the seeds of the liberties we enjoy today. The story of the West is, in part, the often-serpentine working out of these new intellectual currents. That story includes, among other things, monasticism, the split between temporal and spiritual power, and the canon lawyers who began to set out a system of law that honored the rights of the individual.

Rather than the Middle Ages being the time of stagnation of popular imagination, they were a period of important intellectual ferment.

Enforced belief, Siedentop writes, became a contradiction in terms, and eventually the intuitions of Christianity were turned against the institutional church itself, meaning that “secularism is Christianity’s gift to the world.”

This is often obscured because Renaissance humanism looked back favorably to antiquity, and secular authorities had to assert themselves to suppress the violence of the wars of religion of the 16th century. Both trends, according to Siedentop, “suggested that the emergent secularism or proto-liberalism had little to do with the moral intuitions generated by Christianity, but rather that their inspiration should be located in antiquity and paganism.” (It didn’t help that Europe long had a church that was associated with aristocracy and hierarchy.)

Some Muslim writers, Siedentop points out, refer to “Christian secularism,” which isn’t quite the outlandish oxymoron it sounds. “Strikingly,” he writes, “in its first centuries Christianity spread by persuasion, not by force of arms — in contrast to the early spread of Islam.”

Therein lies a tale, one upon which our world depends.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.