"It's getting uncommonly easy to kill people in large numbers," wrote the Christian scholar, novelist, and lay theologian Dorothy L. Sayers in her novel Gaudy Night. "And the first thing a principle does—if it really is a principle—is to kill somebody."

I write this as midnight approaches and the calendar flips to the new Day of Infamy. I am thinking, as are many others, of where I was and what I felt on September 11, 2001, when I first heard that airplanes had struck the World Trade Center's towers.

I was in the basement of the Duke Divinity School's library, attending to my duties as copy editor of Church History, the journal of the American Society for Church History. Adam Zele, the book review editor, hung up the telephone, his face pale. Someone had just called to tell him: Two planes had hit the towers. Another was headed for the Pentagon.

At first I did not believe Adam's news. Surely his informant was wrong—or pulling a sick prank.

But soon I was sitting wordlessly in a darkened film theater at Duke's Bryan student center, with hundreds of other stunned students. In a matter of minutes, we found ourselves bonded into a single disbelieving community of horror.

We watched the footage replayed again and again. We heard the confused reports. Gradually these moved to a consensus: This was the work of terrorists. And terrorists who were not political radicals only, but Holy Warriors. These were men whose hatred of America had boiled up into a blind conviction that God willed their heinous acts.

In those first hours of shock, when all of my insides felt like they had sunk irretrievably to the bottom of my gut, the event seemed a freak—a deed with no parallel.

In the weeks and months after, journalists, professors, and other would-be ...