His real name was Giovanni, Abbot of Struma. He was a strong and early supporter of Frederick Barbarossa, and also supported Antipope Victor IV, who made him a cardinal-bishop of Albano.
He was named successor to Antipope Paschal III by a small number of schismatic cardinals. Callixtus was primarily a bargaining chip of Frederick to pressure Alexander III, and he had only limited geographical support. He resided in Viterbo.
Frederick, when it became politically expedient, after the defeat of Legnano, switched his backing to Alexander, in the 1177 Peace of Venice, abjuring his position on 29 August 1178. Callixtus, besieged at Viterbo by Christian of Mainz, eventually accepted the situation, and at Tusculum, made his submission to Pope Alexander (29 August, 1178), who received him with kindness. Alexander then appointed him Governor of Benevento, where he died a few years later. Some of his obstinate partisans sought to substitute a new antipope, and chose one Lando Di Sezze, under the name of Innocent III.
There is also a pope with this name, Pope Callixtus III.