Saint Theophylactus lived at Constantinople in the eighth century during the time of the Iconoclast heresy. After the death of the iconoclast emperor Leo IV the Khazar (775-780), there entered upon the throne the emperor Constantine VI (780-797). There occurred also a change of Patriarchs: the holy Patriarch Paul, not having the strength to continue guiding the flock afront the powerfully spread iconoclasm, voluntarily resigned the cathedra (784). In his place was chosen Saint Tarasius - at that time an eminent imperial counselor. Under the supervision of the new Patriarch was convened the Seventh Ecumenical Council (787), condemning the Iconoclast heresy. For Orthodoxy a relatively peaceful time began. Monasteries again began to fill with residents.
Saint Theophylactus, a gifted student of Saint Tarasius and with the blessing of this the Patriarch, settled together with Saint Michael in a monastery on the coast of the Black Sea. The zealous ascetics by their God-pleasing labours and intense efforts of prayer were granted by God the gift of wonderworking. By their prayers, during the time of an intense drought when the workers in the field were weakened by thirst, an empty vessel became filled with so much water, that it sufficed for the entire day.
After several years in the monastery they were both consecrated by Patriarch Tarasius to the dignity of bishop: Saint Michael was made bishop of Synada, and Saint Theophylactus was made bishop of Nicomedia.
Heading the Church of Nicomedia, Saint Theophylactus constantly concerned himself about the flock entrusted to him. He built churches, hospices, homes for wanderers, he generously distributed alms, was guardian for orphans, widows, and the sick, and himself attended to those afflicted with leprosy, not hesitating to wash their wounds.
When the iconoclast Leo the Armenian (813-820) came upon the imperial throne, the terrible heresy burst forth with new strength.
But the iconoclast emperor was not able to sway the successor of the holy Patriarch Tarasius, Saint Nicephorus (806-815), who together with the bishops vainly urged the ruler not to destroy churchly peace. Present at the negotiations of the emperor with the Patriarch was Saint Theophylactus, denouncing the heretics, and he predicted a speedy perishing to Leo the Armenian. For his bold prophesy the saint was sent into exile to the fortress Strobil (in Asia Minor). He languished for thirty years until his end, which occurred in about the year 845.
After the restoration of icon-veneration in the year 847, under the empress Saint Theodora (842-855; +867) and her son Michael, the holy relics of Bishop Theophylactus were transferred back to Nicomedia.