Pete Candler is a writer in Asheville, North Carolina.

Kayakers before the Manhattan skyline, forever changed. (David Brown/The Washington Post)

When the regular professor asked me earlier that spring to sub for her in the fall, the date and time seemed an innocuous blank space: Tuesday, Sept. 11, 11 a.m. My assignment: a lecture on St. Augustine’s “City of God,” a fifth-century masterpiece of theology and politics written in response to the fall of Rome.

It wasn’t until I got into the car that day that I heard the news. The top of the South Tower of the World Trade Center, the radio announcer said, was missing. I could neither believe nor disbelieve what I was hearing; there was no reality to which I could connect the crumbling language of NPR anchors straining for barely adequate words. Half an hour later, on a television in the student lounge of Duke Divinity School, I watched as the North Tower exploded in a plume of particles. Students and faculty members huddled together, sobbing on strange shoulders, looking to one another for consolation, wisdom, anything.

I didn’t expect the students to show up when a no-name graduate student guest sub-lecturer was going to talk to them about St. Augustine. They had all just seen an unspeakable atrocity unfold right in front of their eyes. It was probably the best reason anyone ever had to cancel or skip class, to do something more profitable than attend a lecture. What was the point, in the face of such unconscionable, murderous tragedy, of talking about theology?

But when I arrived, the class was packed so full that I had to squeeze past students crowding the doorway just to get into the classroom. They all just sat there, or stood along the back wall, expectantly. I felt exposed, as if I were being taken in for a police lineup. The students — and whatever bystanders had come to stand in the doorway to listen — were not there for me. They were there for St. Augustine.

I had nothing grand for them. I just told them about Augustine. I told them that “City of God” was written in response to a trauma: the collapse of the Roman Empire. Granted eternal dominion by the gods themselves, Rome was supposed to be the Empire without End. So, naturally, it came as a bit of a shock when it fell to pieces overnight, when, not with a bang but a whimper, Rome became just one more empire of dust. Some at the time were blaming Christians for the catastrophe, because Christians worshiped a dying God, seemed to celebrate weakness and claimed as their highest virtue not duty to the nation or force, but love of one’s enemies.

So Augustine set out to write a defense of the “city of God” against these accusations, but it soon swelled into a “giant of a book,” as he called it. “City of God” is a study in opposites: the city of God in contrast to the human, terrestrial city. Augustine’s argument throughout the early books is that, contrary to the high praise Rome lavished upon itself for its commitment to the virtue of clemency, Rome had spectacularly failed, and its temples were not the sanctuaries of humility and mercy Romans wished them to be. In the Roman temples of Juno, he writes, “men were forced into slavery as the property of the enemies who had overcome them”; but in the shrines of the martyrs and churches, “they were conducted to freedom by the merciful.”

In Augustine’s view, it could hardly have been otherwise, because violence is written into the bones of human republics: Rome itself was founded on an original fratricide — Romulus’s murder of his brother, Remus — and the desire for domination over others remains fundamental in every human polity. Augustine finds in Rome the exemplar of “that city which, when it seeks mastery, is itself mastered by the lust for mastery, even though all the nations serve it.”

If we are to take on its own terms the Roman tradition of having gods for everything, Augustine argues, then by their own logic they should have erected a monument to Aliena, the goddess of foreign injustice: “He prays ill,” Augustine writes, “who desires to have someone to hate or fear in order to have someone to conquer.” If, in other words, you need the wickedness of another as the occasion for your own virtue, if you make evil necessary to the good, then — whether you admit it or not — you make a god of evil. The unjust acts of another people or nation cannot spur you to further acts of evil without compromising your own virtues.

This was what the students came to hear from Augustine. They came to hear him argue that when the common interest of a public is not grounded in love for its own sake, and when human rights are not grounded in a universal human calling to love God and one another, then we inevitably serve some other god than the God of Love. We worship at some other altar than that of true mercy and freedom, and above all we end up worshiping an idol whose shifting forms disguise his one name: domination. In our desire for mastery over others, we will merely become slaves to the lust for domination that we mistakenly call freedom.

I talked about this book for 50 minutes, and then took questions that I was not qualified to answer.

I do not recall the specific content of those questions, but I do remember they were asked with an air of immediate urgency. I have never since had such a captive audience, nor one more attuned to the contemporary implications of a very old book. I had tried to give them Augustine, and tried to communicate how revelatory his insights had been to me with some of the same passion that I felt when I first encountered his work. They gave him back to me with an imperative intensity of attention, and a sense that at that moment, it was the most important book in the world.

In those 60 minutes, I realized how much theology matters.