Robert of Arbrissel (d. 1117) once named Cluny among the chief holy places of Christendom--just after Bethlehem, Jerusalem, and Rome. When Peter the Venerable (d. 1156) became the ninth abbot of Cluny in 1122, Cluny had thousands of monks in the mother abbey and her daughter cells, along with hundreds of affiliated houses and dependencies in England, Germany, Spain, Italy and the Holy Land. As a fierce advocate for Cluny against its detractors (which...