KALLISTOS I. (?ca 1300–1363), Patriarch of Constantinople from 1350-1353 and 1355-1363. Author of a Century (100 chapters) on Prayer written around 1360.  He is often confused with the Athonite monk Kallistos Angelikoudis and with Kallistos (II) Xanthopoulos, Patriarch of Constantople in 1397.  Selections from all three are found in the fifth volume of the Philokalia, in several places misidentified and intermingled with texts by one of the other Kallistos.

Kallistos I is said to have lived as a monk on Mount Athos for 28 years as a disciple of St. Gregory Palamas and perhaps also as a disciple of St. Gregory of Sinai.  As patriarch of Constantinople he presided over the Council of Constantinople in 1351 that affirmed the orthodoxy of heychasm and the teaching of Gregory Palamas.  Deposed as patriarch in 1353 for refusing to crown Matthew Kantakouzenos, son of emperor John VI Kantakouzenos, as emperor  with his father, Kallistos returned to Mount Athos. In 1354, after the abdication of John VI. Kallistos returned as patriarch.

Fourteen of his 100 Chapters on Prayer were selected and rearranged in volume 5 of the Philokalia of Nikodimos (Engl.tr. Faber, 2023).

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