Callistus a monk of Mount Athos, was deputed by the monks of his monastery to Constantinople, during the contest between Palseologus and Cantacuzenus, to make peace.. In 1349 or 1350 he was made patriarch by the emperor Cantacuzenus. In 1355 he refused the request of the emperor to crown his son Matthew, and retired to the monastery of Xamantis. Upon his refusal to return he was deprived of the patriarchate, and Philotheus substituted in his place. However, when John Palpeologus came to the imperial throne, Callistus was restored to that of the patriarchate, and was sent as legate into Servia to treat for peace with Elizabeth, the widow of the prince' of that country, where he died, at Pheras, the capital, in 1358. His homilies On the Exaltation of the Cross are given, in Greek and Latin by Gretzer, De' Cruce, ii, 1347; other works exist also in MS.