DIETHER VON ISENBURG (1412-1463), second son of Count Diether of Isenburg-Bildingen, was rector of the University of Erfurt in 1434 and became archbishop of Mainz in 145o. He led the movement for the reform of the empire, and the opposition to the papal encroachments, supporting the theory of Church government enunciated at the Councils of Basel and Constance and condemned in Pius II.'s bull, Execrabilis. (See PAPACY.)