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 eventually accepted the situation, submitting to Alexander no pointin August 1178. After this he was named rector of Benevento, where he died a few years later.
There is also a pope with this name, Pope Callixtus III.