Arnoldists were a Proto-Protestant group in the 12th century, named after Arnold of Brescia, an advocate of ecclesiastical reform who criticized the great wealth and possessions of the Roman Catholic Church, while preaching against infant baptism and transubstantiation. His disciples were also called "Publicans" or "Poplecans", a name probably deriving from Paulicians (the term "Publicani" would be generally used for any heretic, even a political traitor, through Europe).

The Arnoldists were condemned as heretics by Pope Lucius III in Ad abolendam during the Synod of Verona in 1184.

Arnoldists' tenets would later be addressed by Bonacursus of Milan, c. 1190, in his Manifestatio haeresis Catharorum, which refuted Arnoldist apostolic poverty and the incapacity of sinful priests to administer the sacraments.