In The Sorcerer and the Assassin (Brash, Feb.), O’Shea, an expert on 13th-century France, crafts a whodunit centered on the Catholic church’s crackdown on the Cathars.

What about the Cathars led you to write two histories about them?

I lived in Paris for many years, and at dinner parties, inevitably, the subject of the Cathars would come up. I heard that they were fascists, or that they were proto-Nazis. So, I became very curious about them. As I read and researched, I learned how fascinating their history was, and how they were like the road not taken by Christianity. I heard so many contradictory things about them. They’re this mythical people, upon whom many Europeans, especially French people, project their fantasies.

What about their theology was so threatening that the Catholic Church formed the first Inquisition?

In the early 13th century, the church was codifying its beliefs and its institutions. So by the time of our story, the church was a formidable institution, whereas 100 years earlier, it was just a collection of scattered monasteries and ignorant parish priests. The church now saw itself as the ruler of Europe, so any threat had to be eliminated. Cathars were so threatening because they were wildly successful. Catharism was called the Great Heresy of the Middle Ages. Their faith posited that there are two gods: one is ethereal, has nothing to do with this world, and we can join him eventually—and the other is a material god who created the world. Since the material world is evil, this would suggest that the Catholic church and Judaism are worshipping the wrong god, because they’re worshipping the Creator God. And the Cathars went further, stating that all material manifestations of religion are illegitimate. Therefore, church buildings, the sacraments, they’re all nonsense. So it was a pretty extreme position. Cathars also didn’t believe in the paying of tithes, which, of course, infuriated the church.

Why feature them in a whodunit?

My editor in London, Peter Carson, to whom the book is dedicated, said to me after my second Cathar book, “Stephen, you’ve done all this research. Why don’t you have some fun with it?” I just knew stuff from the historical record that is the stuff of great storytelling; there were actual inquisitors assassinated.

What sources could you draw on?

Most repressive apparatuses, whether it be Stalin’s, or Hitler’s, or Napoleon’s, have a tendency to keep wonderful records of everything that they did. And that’s exactly the case with this Inquisition—we know so much about the Cathars simply from the transcripts of the interrogations by the inquisitors.