(antipope Sept. 1168–29 Aug. 1178: d. c.1183)
Originally named Giovanni, he was the third of the imperial antipopes set up during the struggle between Alexander III and Emperor Frederick I Barbarossa (1152–90). He was abbot of Struma, a Vallombrosan monastery near Arezzo, when the imperialist party in Rome elected him to succeed Antipope Paschal III, who died there on 20 Sept. 1168. Almost nothing is known of his earlier life except that he entered the monastery as a monk when still a lad and later became its abbot. Already one of Frederick's keenest partisans in Tuscany, he came out in favour of Antipope Victor IV and against Alexander III after the divided election of 1159. Although Victor created him cardinal bishop of Albano, he was not immediately consecrated but continued as abbot.
The new antipope, who called himself Callistus III, made contact with Frederick through envoys at the diet of Bamberg in June 1169, but the emperor, who had had no direct hand in his election, did not recognize him unambiguously until the negotiations on which he was then engaged with Alexander broke down. Later in the year Callistus, who desperately wanted Frederick to come to Italy to his aid, sent a second embassy to Germany, but although the emperor supplied him with financial assistance he only used him as a means of putting pressure on Alexander. Callistus' following was in fact a modest one in comparison with his predecessors'; only Rome and some districts of the papal state and Tuscany in Italy, and in Germany parts of the Rhineland, acknowledged him as pope. He himself resided chiefly at Viterbo. In 1173 he sent his chancellor, Martin of Tusculum, to Germany to take part in the discussions for an alliance between Frederick and Louis VII of France (1137–79), but nothing is known of his activities in the following years.
When Frederick reached agreement with Alexander at Anagni in Nov. 1176 and, finally, at Venice in July 1177, he abandoned his pope, stipulating only that he should be granted an abbey and that the cardinals created by him should be restored to their previous positions. Notwithstanding the peace Callistus still held out, much to the emperor's annoyance; but when the imperial chancellor restored Alexander to Rome in spring 1178, he could no longer remain at Viterbo but took refuge at Monte Albano, a village near Mentana 23 km from Rome. He finally submitted to Alexander, abjuring his schism, at Tusculum on 29 Aug. 1178. The pope magnanimously entertained him at his table and appointed him rector, i.e. governor, of Benevento, where he must have died at some date before 1184 when another rector is recorded as holding office.
PL ii. 429 f.LP ii. 420 f.Watterich ii. 411 f., 577, 640–42DBI xvi. 768 f. (K. Jordan)EC vi. 610 (P. F. Palumbo)Levillain i. 217 (M. Pacaut)Seppelt iii. 259, 266 f., 271 f.