Making a modern reader care about a 13th-century heretical sect and its destruction is not an easy task, but journalist and translator Stephen O’Shea manages it effortlessly in his new book.

The Cathars rose to prominence 800 years ago, in what is now southern France. They were pacifists who believed that the material world was of no importance, and that the entire panoply of the Catholic Church was a sham. Thus they were locally popular but hated by the religious hierarchy. That hatred led to bloody crusades and a violent inquisition, which slaughtered thousands and wrecked a flourishing feudal economy. The destruction of Catharism also marked a turning point in European history, and led to the establishment of the political unit that we know as France. Even today, Catharism inspires fanatical cults.

The Perfect Heresy (the travelling Cathar priests were called perfects) sets the medieval scene, outlines Catharism, describes the complex religious and political interplay of the crusades, and documents the horrors of the inquisition. Through all this, the book maintains page-turning suspense. The characters face real dilemmas: one count cannot have his excommunication lifted without swearing an oath, but cannot swear an oath while excommunicated.The writing is vivid – a reference to “wine-red bricks” echoes Homer – and the information fascinating: “Kill them all, God will know his own,” for example, was first uttered by a fanatical priest before a massacre at Bezieres in 1209.

Undoubtedly, The Perfect Heresy is light by academic standards – this is a book on medieval history that manages to quote the movie Pulp Fiction – but the tone is ideal for the general reader, and the balance between information and entertainment impeccable. Even O’Shea’s extensive chapter notes are accessibly written and interesting.

The last Cathar perfect died a fiery death in 1321. Is he still relevant? He is to the tourist industry in Languedoc, and to observers of such modern cults as The Solar Temple and Heaven’s Gate, and he will be to readers of this book.