Athonite asceticism and discipleship
For twenty-eight years Kallistos lived as an ascetic at the Magul (Magoula) Skete on Mount Athos, situated opposite the Monastery of Philotheou. He was a devoted follower of Saint Gregory of Sinai, whose teaching shaped his own spiritual formation.
He wrote an account of the life of Gregory of Sinai, preserving the memory of the elder who guided him. During this period he formed a close bond with Saint Ignatios Xanthopoulos, a native of Constantinople; the two monks were described as spiritually unified, both reaching advanced levels of contemplative prayer. Tradition relates that they experienced visions of divine uncreated light, comparable to the experience of the Apostles on Mount Tabor, with their faces appearing radiant.