Elisabeth of Schönau, Saint. [Letter to Hildegard, magistra of Bingen, undated (2)] in Die Visionen und Briefe der heiligen Elisabeth 3.74-78

[Letter from Elisabeth of Schönau to Hildegard, magistra of Bingen]
Elisabeth encourages Hildegard in her opposition to the Cathar heresy with her own attack on Cathars and those in the church who fail to stop them, connecting their failure with the problem of simony.
Online at Epistolae (See also Bibliographia record).