‘The power of the Church ought to be aroused to obliterate the wickedness of various heresies, which in modern times have begun to sprout in many parts of the world.’ The opening words of this papal decretal of 1184 articulate a view widely held among churchmen of the decades around 1200 about the danger of heresies, which, combined with pastoral deficiency, constituted an extraordinary crisis for the Latin Church. The groups denoted here by the words wickedness and heresies regarded themselves as good and Christian. However, they have left virtually nothing of the theological libraries and administrative archives which historians would use to construct accounts of the church or religious orders. When looking at these groups this chapter is therefore forced to adopt much of the language and viewpoint of the church. It has to begin with the principal archaeological remains, namely the large numbers of manuscript books which contain the church’s views of the topic, and then ask how these groups were fashioned and reshaped in these texts.

This means looking at the contents of the libraries of cathedral chapters and religious houses, and then the works produced by and for theological and canon law teaching in the schools and universities. Copies of books of the Bible included passages held to denote heretics and their characteristics, such as wolves in sheep’s clothing, while there was an older deposit of patristic writing against early church heretics, especially the dualist Manichees, and important definitions of church, sect and heresy in what was the Middle Ages’ dictionary, Isidore’s Etymologies.