Unlike the crusades fought against Islam in the eastern Mediterranean, the Albigensian Crusade was directed at fellow Christians within Western Europe, the Cathars of Languedoc, whose dualist teaching that the material world was the creation of an evil power stood outside Catholic doctrine on creation and the incarnation. Papal legates had preached and debated against Cathar belief in the region for decades without dislodging it, in part because local nobility, including Raymond VI of Toulouse, tolerated or protected Cathar communities within their lands.
The murder of the papal legate Pierre de Castelnau in January 1208, for which Innocent III held Raymond VI responsible, gave the papacy grounds to call a crusade rather than continue preaching. Innocent offered the same indulgence granted to crusaders in the East to anyone who took up arms against the Cathars and their protectors, and a largely northern French army, along with contingents from elsewhere, assembled in 1209 under the nominal leadership of the papal legate Arnaud Amalric. Cathar teaching held that a good, spiritual God stood apart from an evil creator responsible for the material world, and that the perfecti, an ascetic elite within the movement, alone underwent the consolamentum that freed a soul from the cycle of rebirth into matter; ordinary believers, the credentes, were not required to live by the same austerity until near death.