Gibbon Chapter LIV: The Paulicians and the Reformation
At 18 pages this is one of the shortest chapters in the entire Decline and Fall - possibly even the shortest; I haven't gone back and counted. Gibbon outlines the early history of the Paulician heresy in Anatolia in the ninth century, links them to the Albigensians (who we now tend to call the Cathars) in the thirteenth century, and then explores the benefits of the Reformation.