According to the centuries-old rules of the Roman Catholic Church, sin-reduction is a two-step process. Guilt is absolved through confession and prayer, but punishment — on earth or in purgatory — can be avoided through indulgences, an ancient form of church-granted amnesty that critics deride as a shortcut to salvation. The door for indulgences is not always open, though, and for years after the Vatican Council reforms of the 1960s, they were rarely offered — until 2000, when Pope John Paul II started using them to attract pilgrims to World Youth Day. Today, Pope Benedict XVI put out the latest...