John Cassian (kăsh´ən) (Johannes Cassianus), 360–435, an Eastern Christian monk and theologian who brought Eastern spirituality to the West. Cassian toured the ascetic monastic settlements of Egypt before he was driven from the East during the controversy over the theology of Origen. He settled at Marseilles (415) and established religious houses for men and for women. He was attacked for Semi-Pelagianism (see Pelagianism), but he was trusted in Rome. His Conferences, a record of his earlier experiences with famous abbots and ascetics in Egypt, and his Institutes, a treatise on monasticism, had a critical influence on Western monasticism, especially in matters of ascetic and mystical life. He wrote against Nestorianism.
See study by O. Chadwick (2d ed. 1968).