Monastic Life on Athos
Callistus passed approximately twenty-eight years as a monk on Mount Athos, residing at the Skete of Magoula near the Monastery of Philotheou. There he came under the spiritual direction of Saint Gregory of Sinai, whose biography he would later write, and he is also numbered among the disciples of Saint Gregory Palamas. He founded the Monastery of Saint Mamas on the island of Tenedos, near the Dardanelles.
His long formation in the hesychast tradition of Athos shaped his later defense of that tradition from the patriarchal throne and informed his writings on monastic prayer.
First Patriarchate and the Defense of Hesychasm
Callistus was elected Ecumenical Patriarch on June 10, 1350, succeeding Isidore I. The most significant achievement of his first patriarchate was the convening of a synod in Constantinople in 1351 that finally established the Orthodoxy of hesychasm, defending the mystical theological tradition against opponents such as Barlaam of Calabria. He and subsequent patriarchs worked to promote Palamite doctrine across the Eastern Orthodox jurisdictions, though this effort met resistance from the Metropolitan of Kyiv.
In 1353 he was deposed after refusing to crown Matthew Kantakouzenos as co-emperor alongside John VI Kantakouzenos.
Second Patriarchate
After John VI Kantakouzenos abdicated in 1354, Callistus returned to the patriarchate, serving a second term that ran from 1355 to August 1363. He reorganized the parish system under patriarchal exarchs, strengthening the administration of the Church.
In this period he excommunicated the Serbian tsar Stefan Dušan for establishing an independent Serbian patriarchate, and in 1355 he declared single-immersion Latin baptisms improper and requiring re-baptism.
Writings
Callistus collaborated with the monk Ignatius Xanthopoulos on a 100-section ascetical text, a 'Century' on hesychastic monastic practices, which was later incorporated into the Philokalia. He also composed a hagiography of Saint Gregory of Sinai, written around 1351, and a hagiography of Saint Theodosius of Tarnovo.
Death
Callistus died in August 1363 while serving as an ambassador for Emperor John V Palaiologos, having traveled to Serbia to seek military aid against the Ottomans. Contemporary accounts record that Saint Maximus of Kapsokalyvia prophesied his imminent death.
