Catholic Encyclopedia (1913)/Diether of Isenburg
Archbishop and Elector of Mainz, b. about 1412; d. 7 May, 1482, at Aschaffenburg. He studied at the University of Erfurt, of which he became Rector in 1434. At the age of sixteen he was already canon at the cathedral of Mainz and, somewhat later, held prebends in the cathedrals of Cologne and Trier. In 1442 he became provost in the collegiate churches of St. Victor and St. John in Mainz, and in 1453 custos of the cathedral chapter in the same city. In 1456, Diether aspired to the vacant See of Trier, probably attempting to gain votes through simony; but the majority of the voters decided in favor of John of Baden. But after the death of Dietrich of Erbach, the Archbishop and Elector of Mainz (d. 6 May, 1459), Diether's ambitious aspirations were realized. Probably through simony he was elected to this see on 18 June, 1459, and entered upon his office without awaiting the approval either of pope or emperor. Pope Pius II was then holding a congress at Mantua with the object of influencing the princes of Europe to undertake a crusade against the Turks, and was greatly disappointed at the small number of princes that appeared at the congress. Upon hearing that Diether did not intend to come to Mantua, but had sent envoys to obtain the papal confirmation, he sent word to him that, if he desired the Bull of confirmation and the pallium, he would have to come to Mantua in person. By pleading sickness and lack of funds, Diether finally prevailed upon Pius II to send the Bull of confirmation and the pallium, but only after promising under oath to appear personally before the papal court within a year and to pay the annates, which amounted to 20,550 Rhenish florins. When Diether kept neither of these promises, the Curia punished him with minor excommunication. Angered at this act, he convened a diet at Nuremberg in February, 1461, at which he made a formal appeal to a future general council, despite the fact that Pius II in his Bull "Exsecrabilis" (18 Jan., 1460) had condemned such appeals as heretical. When all the efforts of the pope to bring about an amicable settlement were frustrated by the rebellious archbishop, the pope excommunicated and deposed Diether in a Bull dated 21 August, 1461, and appointed Adolf of Nassau in his place. Diether tried to retain his see by arms but was compelled to yield to the superior forces of Adolph of Nassau in October, 1463. After submitting to the new archbishop and apologizing to the papal plenipotentiary, Pietro Ferrici, he was absolved from the sentence of excommunication. Upon the death of Adolf of Nassau, on 6 Sept., 1475, Diether was again elected archbishop of Mainz and received both papal and imperial approbation. His past misfortune had made him a better man. He enforced strict ecclesiastical discipline among the clergy, watched carefully over the integrity of faith, and worked strenuously for the advancement of commerce and education. At the northern end of Mainz he erected the Martinsburg as an archiepiscopal residence, and in 1477 founded the University of Mainz, which continued to exist until 1798.
MENZEL, Diether von Isenburg (Erlangen, 1867); Annalen des Vereins fur nassauische Alterthumskunde (Wiesbaden), X and XII; PAST0R, Geschichte der Papste (2 ed., Freiburg 1894), II, 122 sqq., tr. ANTROBUS (London, 1894), III, 164 sqq.; GUNDLACH, Hessen und die Mainzer Stiftsfehde, 1461-63 (Marburger Dissertation, 1898).