Kallistos was elected to the throne of the see of Constantinople in June 1350, succeeding Isidore I. In 1351, he convened a synod in Constantinople that finally established the Orthodoxy of Hesychasm. Kallistos I and the ecumenical patriarchs who succeeded him mounted a vigorous campaign to have the Palamite doctrine accepted by the other Eastern patriarchates as well as all the metropolitan sees under their jurisdiction. However, it took some time to overcome initial resistance to the doctrine. One example of resistance was the response of the Metropolitan of Kiev who, upon receiving tomes from Kallistos that expounded the Palamist doctrine, rejected the new doctrine vehemently and composed a reply refuting it. According to Martin Jugie, contemporary historians depict Kallistos as a "doctrinaire and brutal man whose persecuting zeal it was necessary to restrain." In 1353, Kallistos refused to crownMatthew Kantakouzenos, son of emperor John VI Kantakouzenos, as emperor with his father and, as a result, was deposed. After his deposition, Callistus returned to Mount Athos. In 1354, after John VI abdicated, Kallistos returned as patriarch. After his return, Callistus worked to strengthen the administration of the patriarchate. He reorganized the parish system of churches under the surveillance of a patriarchal exarch. He also strove to strengthen patriarchal control over various Orthodox church jurisdictions, even to the extent of excommunicating Stephen Uroš IV Dušan of Serbia, for establishing the Serbian archbishop as an independent patriarch. In 1355, Patr. Callistus of Constantinople wrote to the clergy of Trnovo that those Latins who had baptized by single immersion should be re-baptized. He called the baptism by one immersion most improper and full of impiety. His view was based on the Apostolic canons which clearly state that those baptized by one immersion are not baptized and should be re-baptized.
Death
Patr. Callistus died in 1363 while he was en route to Serres as a member of the embassy of emperor John V Palaiologos seeking aid from Helena of Bulgaria, Empress of Serbia against the Ottoman Empire. Of note is that St. Maximus of Kapsokalyvia prophesied the death of Patriarch Callistus. On his way to Serbia, Callistus traveled through Mount Athos. Seeing him, St. Maximus said, "This elder will never see his flock again for behind him is heard the funeral chant: "Blessed are the undefiled in the way".