Pagans thought that the collapse of their beliefs would mean the collapse of Rome. Many 21st-century conservatives believe something similar about the erosion of Christian values: that the liberties of our open society are parasitical on our Christian inheritance and that when that inheritance collapses, civilization will, too.

Ms. Delsol does not see things quite that way. The ethics of the Christian age, she notes, were shot through with unacknowledged borrowings from the pagan values Christianity replaced. (Consider stoicism or the Hippocratic oath in medicine.) In the same way, today’s post-Christian progressivism comes with a large helping of Christianity. Why use Christian matrimony to unite gay couples, for example, rather than a new institution less wrapped up in Christian values? Because that is just the piecemeal way that civilizational change happens.

So if another civilization comes to replace Christianity, it will not be a mere negation, such as atheism or nihilism. It will be a rival civilization with its own logic — or at least its own style of moralizing. It may resemble the present-day iconoclasm that French commentators refer to as le woke. (The term means basically what it does in English, except that French people see wokeness as a system imported wholesale from American universities and thus itself almost a religious doctrine.)