Context:

Tom cites this book as a brilliant account of the Albigensian Crusade, quoting from it multiple times throughout the episode including descriptions of the violence and a story about a ghost appearing from purgatory

Context:

Referenced as written by 'the great historian of this Christian revolution,' discussing the parallel between the Albigensian Crusade and the witchcraft panic of the 17th century

Context:

Mentioned as a modern book that cast the Cathars as feminists, discussed in the context of how the Cathars have been romanticized in the 21st century

Context:

Referenced throughout the episode series as the starting point for discussing Cathar conspiracy theories, including the idea of the Cathars as a secret society protecting the bloodline of Jesus and Mary Magdalene