More than a century after Nietzsche’s announcement of the death of God, the Old Boy not only lingers on but has yet to enter palliative care. No one languishing in a truly moribund condition could possibly attract so many enemies. Consider, for instance, the American commercial book trade’s answer to the Three Tenors: the Three Atheists—Richard Dawkins, Christopher Hitchens and Sam Harris. Sure, we’ve heard it all before, but legions of readers still can’t get enough of it.

None of these enemies of God, however, is remotely as sophisticated—or as ambivalent—as Mark Lilla. One may find his title, The Stillborn God, repellent and the ambition of his subtitle, Religion, Politics, and the Modern West, presumptuous, yet this is one of those rare books that repays careful reading. It has much to teach about where we of the West are today, and the strange journey by which we have arrived here. There is no comparison between this highly erudite and demanding work, which originated in a set of academic lectures, and those of the aforementioned popular screedists.

Which raises the question of the sanity of the publishers at Knopf. The Stillborn God consists almost exclusively of abstruse accounts of Christian theology, on the one hand, and the ruminations of Great Modern Thinkers, on the other. It contains not one light or easy moment. Did those corporate honchos really think that somewhere there lurked a general audience for such a book? And wouldn’t it be heartening if they were right?

Lilla’s theme is what he calls the Great Separation and its consequences, intended and not. Americans are familiar with this development as the separation of church and state. According to Lilla, however, we take this arrangement so much for granted that we no longer grasp its significance. The Great Separation wasn’t primarily a matter of institutional accommodations; it constituted a fundamental transformation of the Western view of the world and the role of religion in it.

The key notion to grasp is political theology, the central event being the West’s rejection of it. For most of the world’s peoples throughout history, and for many non-Western ones still today, the ultimate ground of political authority has been the will of God, as interpreted by those human beings held to be licensed by Him to do so. While this cast of thought may strike liberal Westerners as benighted, this merely confirms (says Lilla) how much we have forgotten about our own past and how cavalier we are inclined to be about our possible...