|19th century bust of Arnold.|
Arnold was born about 1190, in Lombardy in northern Italy. He joined the Augustinians, whose frugal ways clashed with the activities of the increasingly powerful popes. He supposedly studied at the University of Paris under Peter Abelard. Arnold and Abelard both were outspoken about the temporal power of the papacy, but they lost the debate at the Synod of Sens in 1141. Abelard gave in, but Arnold kept up his vocal condemnation of the popes. He was condemned by Pope Innocent II (mentioned here and here), and fled to Zurich.
After Innocent's death, Arnold reconciled with Pope Eugene III, but when he returned to Italy and found that Rome had changed its political structure and refused to allow Eugene to return, Arnold sided with Rome and quickly rose to a position of authority (rather counter to what he objected to about the papacy). He preached that priests who owned property gave up their qualifications to administer the sacraments.
Eugene in exile excommunicated Arnold, but even when Eugene managed to return to Rome, Arnold continued to wired political power in opposition to papal policies.
The next pope, Adrian IV, was not as mild-mannered and easily pushed around as Eugene: he took control of Rome in 1155 with the help of Holy Roman Emperor Frederick Barbarossa and forced Arnold into exile, where he was picked up by Barbarossa's forces and forced into a trial. He refused to renounce any of his positions—even when faced with execution—and he was hanged for rebellion (not heresy, curiously) in June 1155. His body was burned and the ashes thrown into the Tiber River to prevent his tomb from becoming a focal point for sympathizers who would consider him a holy martyr.