Little is known of John’s life before he took office as pope, except that he was born in Tuscany and that his father was Constantius. He was elected a week after the death of his predecessor Hormisdas.
Thanks to the recent reunification of the Eastern and Western Churches under Hormisdas, relations were very good with the Byzantine empire, but for the same reason they were strained with Theodoric, king of the Ostrogoths. The Byzantine emperor Justin persecuted heretics with enthusiasm, and he issued an edict against Arianism in 523. Theodoric, an Arian, distrusted the papacy’s affinity to Justin, and he pressured John to go to Constantinople and convince the emperor to withdraw the edict.
John did indeed go to Constantinople and was well-received, but the edict was not withdrawn. Upon his return to Italy, Theodoric had John arrested and imprisoned in Ravenna. Worn out by his journey and probably starved, John died in prison soon after. Pope St. John I is honored as a martyr.