"Irony" seems a concept invented for such a situation as this: The man historian Christopher Dawson once called the most influential Englishman who ever lived is the patron saint of … Germany.

And, as journalist Uwe Siemon-Netto has recently reminded us, the 60th anniversary of D-Day is also the 1250th anniversary of this man's death.

There is one more layer of seeming irony in this story of the man who evangelized Germany and set the stage for Western Christendom: he was a monk.

Before we get to the man himself, we should think about this fact. Every modern person knows that monks lived out their lives in cloistered irrelevancy, too busy with the inward pursuit of holiness to do much to change the course of history—right? Wrong, of course. Thomas Cahill has busted that myth with his paean to Irish monasticism, How the Irish Saved Civilization. But it lives on, perpetuated by Hollywood and Madison Ave.

In fact, in the centuries that followed the fall of Rome to the barbarian tribes, it was the monks who did most to convert the conquerors to the religion of the conquered. Monastics were used by Gregory the Great (540-604) and the Roman Christians. The most famous of these was the "apostle to England," Augustine of Canterbury, who missionized England in 597-604/5. And Celtic—that is, Irish—monks did much to bring the Christian faith to the European continent. The star here was Willibrord, who led a highly successful mission in the area of modern-day Belgium and Holland from 690 on.

Typically, a cluster of 10 or 12 pioneer monk-missionaries would come to a new—and thoroughly pagan—area, set up a church, bring people into the fellowship, teach, and train leaders. During their mission, they would plant ...