John I was elected pope in 523. King Theodoric the Great, the Arian Christian Ostrogothic ruler of Italy, sent him as ambassador to Constantinople to plead with the Eastern Emperor Justin I for tolerance for Arians within the Byzantine empire — under threat that Theodoric would otherwise persecute the Catholic Italians.
John traveled to Constantinople, where he was the first reigning pope ever to set foot in the imperial city. He was received with great honors. Justin refused his petition for the Arians but consented to confirm the Catholic Italians' faith. John returned to Ravenna in 526; Theodoric, suspecting treason, imprisoned him. He died in prison after a few weeks of confinement — by tradition, of starvation.
His body was brought to Rome with great public mourning and buried in St. Peter's. He is venerated as a martyr — the second pope after Anacletus to die under unjust imperial detention.