patriarch of Constantinople (John IV) and mediator of theological disputes between the Orthodox and monophysites (q.v.). He reinforced Constantinople's preeminence among patriarchal cities in the Eastern Church by assuming the contested title of ecumenical patriarch. His sobriquet “the Faster” derived from his meagre use of food.

John's devout and conciliatory nature attracted him to Patriarch John III Scholasticus, who ordained him deacon at the cathedral of Hagia Sophia and appointed him to minister to the needs of the poor. Favoured by the Eastern Roman emperor Tiberius II, John became patriarch of Constantinople in 582. A dispute over the title ecumenical patriarch, appropriated by John at a synod in 588 and continued by his successors to the present, evoked charges from the 6th-century popes Pelagius II and Gregory I of usurping spiritual authority. Historical and linguistic evidence exists, however, to show that the title had precedent in both East and West; it signified a spiritual primacy relative to the political boundaries of the Byzantine Empire, whose capital, Constantinople, was in ecclesiastical rank second only to Rome. The Greek Byzantine Church, impressed by his ascetical and charitable character, proclaimed him a saint.