Discipleship and Iconoclasm
The defining feature of John's life in the sources is his discipleship to Gregory the Decapolite. He received the monastic habit from Gregory at Thessalonica, and his obedience is said to have caused his teacher to glorify God. The two are consistently presented together with Joseph the Hymnographer, who would later become one of the most celebrated hymnographers of the Byzantine Church.
The synaxarion places John's public activity in the context of the second period of Byzantine Iconoclasm, renewed under Leo V the Armenian. The sources state that John, alongside Gregory, openly preached the veneration of the holy icons in Constantinople and endured persecution for several years rather than abandon the Orthodox position.