By 814 John had become an iconoclast, and Emperor Leo V the Armenian chose him to head a committee to collect patristic texts supporting this theological position in preparation for the Synod of 815, which reinstated iconoclasm.
John was rewarded for his efforts by being appointed abbot of the prestigious monastery of Sergius and Bacchus (now the Little Hagia Sophia), where recalcitrant iconodules were re-educated.
John was known for his learning (hence his nickname Grammatikos) and for his persuasive rhetoric in the endless debates that are a favorite subject of hagiographic sources reflecting the second period of the Iconoclasm.
John was also charged with tutoring the future Emperor Theophilos during the reign of his father, Michael II, and is credited with instilling strong iconoclastic sympathies in his pupil.
He did, however, bring back a plan of the Abbasid palace in Baghdad for his emperor's amusement, and oversaw the construction of a similar structure in Bithynia.