One of the most dramatic transformations of culture in the history of Western civilization was accomplished by humble, poor, foreign missionary teachers. Irish monks—C. S. Lewis’ forebears—trudged their way across Scotland, England, and the European Continent. And in their wake they left a lasting legacy of faith, of learning, and, indeed, of civilization. Over the centuries between A.D. 550 and 1300, scores of Irish monks left their homeland and spread out across Europe, preaching the...