Arnold Of Brescia (Arnaldo da Brescia), a religious reformer, born at Brescia in Italy about the beginning of the 12th century, executed at Rome in 1155. He first appears in history as a scholar of Abelard, and was distinguished for eloquence. Returning from France to Italy, he attacked the luxury, venality, indifference to religious duties, and degrading worldliness of the clergy. His special doctrine was the antagonism of the church to the world. He held that the same man ought not to hold secular and religious office. This doctrine speedily made for him a party. Disturbances broke out, the clergy protested, the bishop of Brescia became alarmed, a complaint was sent to Rome, and at the council of the Lateran in 1139 Arnold was condemned as a disturber of the peace, forbidden to preach, and banished from Italy. His party, however, was not annihilated, nor his influence destroyed. In France, where he went to visit Abelard, whose name had been joined with his in the sentence of condemnation, and in Switzerland, where he preached for some years, he gained many adherents. Meanwhile, a bold application of his principles had been attempted in Rome itself.

The demands of the papal see excited a popular movement (1143), and secular authorities were appointed to govern the state, while the pope, Innocent II., was restricted to the exercise of spiritual authority. This change in the national government being opposed by Innocent and his successors, a revolt broke out in 1145, and Pope Eugenius III. was forced to leave the city. Arnold went to Rome and assumed the direction of the popular movement; but the license of rioters hindered his plans, reaction came, one by one his reforms were nullified, and the unfortunate murder of a cardinal in the street enabled Pope Adrian IV. to turn against this alleged disturber of the peace and enemy of the church the sympathies of the populace. Arnold was with his friends driven from the city, and sought refuge with some noblemen of Campania. When the emperor Frederick Barba-rossa came to Rome to be crowned, the pope asked him to have Arnold arrested. The order was given and executed, and Arnold was strangled, and his body burned and thrown into the Tiber. - The character of Arnold has been variously represented.

Baronius calls him "the father of political heresies." The truth appears to be that he was a great reforming spirit, who fell into many errors and excesses, but whose leading idea was to renovate the clerical order after the apostolic model. Baptist writers claim him as one of the forerunners of their faith, the denial of infant baptism being among the charges against him at the Lateran council of 1139. A sect called Arnoldists existed in Italy for some time after his death. They were condemned at the council of Verona in 1184, and the name occurs in a law against heretics of Frederick II. (1224).