Who are the Messalians and what is Messalianism?
In the history of Christian sects, there were a number of dualist groups who identified themselves as special or set apart in some sense, and often in reference to a life of prayer. In the history of the Christian Roman Empire of the East there were the Euchites or Messalians (c. 4th - 7th centuries), and the Bogomils (c. 10th - 14th centuries), their names meaning “praying people”. These groups had certain traits in common, among which were an emphasis on unceasing prayer, by which the devil was constantly repelled; the rejection of all images of the Divine; and belief in their ability to experience a vision of the essence of God through their natural, physical senses. They were itinerant and practiced complete abstinence for long periods of time, after which they would live equally long periods of complete lascivious behavior, revealing thereby their gnostic world-view: that they were, due to their higher knowledge and divine status (by nature), untouched or unaffected by the physicality of the evil world.
In St. Gregory’s times (i.e. in the thirteen and fourteenth centuries), the anti-Hesychasts accused the Hesychasts of being Messalians (following the prevalent custom of naming peoples and heresies by the older name), because the Hesychasts practiced unceasing prayer, and they suspected them of iconoclasm, because they rejected images and the imagination in prayer.
For further information on the Messalians and Bogomils, see also my Notes in Christopher Veniamin, ed., Saint Gregory Palamas: The Homilies (Dalton PA, 2022), p. 545, n. 103; and p. 597, n. 554.