Most everybody has heard about the Crusades. Generally, what people mean when they speak of the Crusades is the series of wars fought between Christianity and Islam. However, within the whole area of Crusading one particular episode sticks out, the Albigensian Crusade. The first Crusade waged wholly against fellow Christians, if not the last. It is also significant for a phrase that originated during the Crusade that has remained in use since that time and last but not least, it was the impetus for the founding of the Inquisition.
I first heard of the Albigensian Crusade in high school and it piqued my interests then. I actually wrote a paper about it in my freshman world history class in college. The origins of the Crusade lie in the growth and spread of what has become known as the Cathar Heresy. The Cathars were a radical sect that rejected Roman Catholicism in its entirety believing that the church was irredeemably corrupt. The Cathars, who were centered in an area of southern France called Languedoc grew rapidly in the 12th century and were seen as a serious threat to the church. This threat occasioned the call for a Crusade against it by Pope Innocent III in 1209, the Pope reissued these calls every year until the Cathar Heresy was stamped out. An example of the 1215 Call to Crusade can be found here.
The Albigensian Crusade itself was very popular within France, much more so that Crusade to the Holy Land. For one thing, Languedoc was much closer than Outremer, and a 90 day Crusading vow was all that was needed to obtain absolution. The Crusade was very popular and initially very successful with the major towns of Beziers and Carcassonne falling the first year. Raymond VI the Count of Toulouse, who implicitly supported the heretics was excommunicated and only reconciled to the church after meeting very harsh terms. The Crusade dragged on for twenty years until the King of France got involved in 1229 and decisively stamped out the Cathars incidentally expanding royal power into the south of France.
The event in the Crusade that has echoed down to the present happened at the siege of Beziers in 1209. The crusaders had conquered the town and trapped many of the citizens both heretic and non-heretic inside the main church downtown. The papal Legate Bishop Arnaud Amaury was asked what to do since the crusaders could not tell the heretics from the Christians within the church. Amaury responded with a phrase familiar to soldiers down the present “Kill them all, for God will surely know his own.” Upon hearing this, the crusaders set the church on fire and killed everyone inside innocent and heretic alike.
Some further reading on the Albigensian Crusades can be found at:
The Crusading Encyclopedia, The Albigensian Crusades, De Re Militari, and the Boise State University History of the Crusades webpage.