Though almost no one refers to the entire medieval period as the "dark ages" anymore, the years from the fifth through the fifteenth century were often gloomy. Western Christendom battled Islam to the south and east, barbarians to the north and west, and plagues, famines, and feudal warfare at home. Then there was the problem of heresy, sprouting both local varieties and exotic foreign species.

Of the former strain, thirteenth-century Catharism was viewed as perhaps the most poisonous. At least it elicited the fiercest response. First the Cathars weathered a particularly vicious crusade: 20,000 people were slaughtered in the city of Beziers alone after the monk in charge of the assault, when asked how to distinguish heretics from Catholics, replied, "Kill them all, God will know his own." Surviving Cathars then became the original targets of the Inquisition.

The Cathars receive much nicer treatment from journalist Stephen O'Shea, who traces their history in his lively new book The Perfect Heresy: The Revolutionary Life and Death of the Medieval Cathars (Walker & Company). He labels them "a pacifist brand of Christianity embracing tolerance and poverty" and groups them with the pre-Protestant Waldensians as legitimate challenges to an authoritarian church. However, he also admits that they were "well and truly heretical, by every definition except their own." Let me just say that I appreciate his narrative more than his judgment.

There's no doubt that the Cathars, who called Languedoc (now southern France) home, were revolutionary. Dissatisfied with a church accumulating temporal wealth and authority, they adopted a gnostic dualism: matter was evil, and salvation lay in sloughing off the mortal coil after achieving perfection, ...