pope from 523 to 526. He ended the Acacian Schism (484–519), thus reuniting the Eastern and Western churches by restoring peace between the papacy and the Byzantine emperor Justin I. He also ratified the Alexandrian computation of the date of Easter, which was eventually accepted throughout the West.

Unfortunately the reunion ended friendly relations between the papacy and Theodoric the Great, the Ostrogothic ruler at Rome. Theodoric mistrusted the rapprochement, and his hostility flared when Justin took harsh measures against the heretical Arian (Arianism) sect in the Byzantine Empire. Theodoric was himself an Arian Christian, and in 525 he ordered Pope John to negotiate a withdrawal of Justin's edict. John was the first pope to go to Constantinople, but his attempt there to obtain a reversal of Justin's policies was only partly successful. After he returned to Ravenna (526), Theodoric had him cast into prison, where he soon died, probably of starvation. He is honoured as a martyr.