(antipope 25 June 1080; 24 Mar. 1084–8 Sept. 1100)
Born at Parma c.1025 of a family related to the counts of Canossa, Guibert was in the entourage of Bishop Cadalus of Parma in 1052, came to the German court in 1054 and, on the nomination of Empress Agnes, was imperial chancellor for Italy 1058–63. As such he was present in Jan. 1059 at the synod of Sutri at which Nicholas II anathematized Antipope Benedict X; the claim that the reference to the king in the electoral decree of that year was a compromise suggested by him is open to question. On Nicholas's death, when the reform party at Rome elected Alexander II, Guibert was one of the moving spirits in the election of Cadalus as Antipope Honorius II at Basle in Oct. 1061. In 1072 he was nominated archbishop of Ravenna by King Henry IV (1056–1106); Alexander II was at first reluctant to consecrate the backer of his rival, but was persuaded to do so by his archdeacon, Hildebrand, after Guibert had taken the oath of allegiance. When Hildebrand became pope as Gregory VII in 1073, Guibert at first cooperated with him, but soon moved into the camp of his enemies. In 1075 Gregory suspended him for failing to appear at his Lenten synod; in Feb. 1076 he was excommunicated for his part in the meeting of Lombard bishops which had purported to depose Gregory. When the final break between Henry IV and Gregory came, the king had Guibert elected pope at Brixen in June 1080. When Henry at last took possession of Rome four years later, the Roman clergy and people elected Guibert pope, and he was enthroned in the Lateran basilica on 24 Mar. 1084 with the style Clement III. On 31 Mar., while Gregory still held out in Castel Sant'Angelo, he crowned Henry as emperor in St Peter's. The approach of Robert Guiscard, duke of Apulia (c.1015–85), however, with a Norman army obliged Henry and Clement to abandon Rome. The antipope betook himself to Ravenna, of which he had remained archbishop and which from c.1080 onwards he organized as a centre of pamphlet war against Gregory and the Gregorian party: he returned there frequently.
Clement impressed friends and foes alike as a man of irreproachable character, first-rate ability and education, and remarkable eloquence. He was no tool, as has sometimes been supposed, of the emperor, but developed policies of his own, and was personally responsible for effective anti-Gregorian propaganda. Because of the support he enjoyed among the clergy (thirteen of the cardinals had come over to him and eventually seventeen of the 28 cardinal priests were on his side, though only one cardinal bishop) and the people, he was able to return to Rome and exercise his role as pope throughout Victor III's reign and most of Urban II's. He made strenuous efforts, with differing degrees of success, to have his legitimacy recognized not only in Germany and north Italy, but in England, Portugal, Denmark, Hungary, Croatia, and Serbia; and he negotiated with both Archbishop John II of Kiev and the eastern emperor and patriarch in the interest of union. No opponent of reform, he legislated at a Roman synod in 1089 against simony and clerical marriage and in favour of clergy living in common; he also condemned the uncanonical practice of the Gregorians of treating the sacraments of schismatic priests as invalid. He was indirectly responsible for the development of the college of cardinals, for he allowed so much influence to the cardinal priests who came over to his side that Urban II had to treat the ones who supported him with like consideration. In the mid-nineties his power and authority began to wane; he was driven out of Rome by the Pierleoni family in 1098, and Castel Sant'Angelo, the last Clementine stronghold in the city, fell to Urban II on 24 Aug. of that year. On the accession of Paschal II in 1099 he prepared to renew the struggle, but was ejected from Albano by Norman troops. He died at Civita Castellana, 57 km north of Rome, in Sept. 1100.