After winning the Investiture Controversy, the Church faced discontent among the faithful, who viewed its attachment to wealth as increasingly distant from the Gospel’s message. The Cathars emerged as the most outspoken critics of Catholicism in the West before the rise of Protestantism.  

Originating in Occitania around 1150, they were named after the Greek word καθαρός (katharós), meaning "pure," due to their connection with Gnosticism & the Manichean dualism that had taken root in the Balkans & Thrace during the 10th century, particularly among the Bogomils. They referred to themselves as "Good Men." Their doctrine was founded on opposition between the spiritual world, created by God, & the material world, which they believed was created by Satan to entrap & corrupt souls. According to their belief, the Creator in Genesis was actually the devil - an anti-God figure - and all material things were inherently evil.  

The most radical Cathars not only rejected wealth but also condemned forms of worldly pleasure, especially sexuality. They preached an ascetic self-extinction of humanity as the ultimate liberation of souls from their bodily prisons. To achieve salvation, the "good" had to recognize the horror of earthly life, renounce sex, and allow themselves to die of starvation.  

From Occitania, their religious centers spread to the Catalan counties of the Pyrenees, Provence, & Lombardy, attracting the support of feudal lords. They gathered in fortified villages known as castrum settlements, which featured walls, towers, & castles. 

Dominic of Guzmán (later Saint Dominic) and Francis of Assisi opposed the Cathars' bleak worldview, though they acknowledged the power of their call to evangelical poverty. The Church attempted a peaceful & tolerant approach by sending preachers to counter their influence. In 1208, papal legate Pierre de Castelnau was assassinated. The following year, Pope Innocent III declared a crusade against the French Cathars.  

The massacres that ensued amounted to what many consider a genocide, leading to the decline of Occitan culture. Yet, it took more than a century & the efforts of the Inquisition to fully eradicate Catharism from Europe.