Traditional Christian virtue and vices like abstinence, gluttony, and sloth make many of us bored or uncomfortable. At their best, these words sound dead or confusing, like incomplete fossils that belong to a distant past awkwardly enshrined in a museum. At worst, they signify a prejudiced past, when these words were wielded like weapons. Yet in medieval writing, the language of the virtues and vices was powerful, lively, and delightfully...