patriarch of Constantinople, whose deposition caused a serious schism in the Byzantine Church. He took the name Arsenius on being appointed patriarch of Nicaea in 1255 by the Byzantine emperor Theodore II Lascaris. In 1259 he crowned John IV (John IV Lascaris), Theodore's son and legitimate heir, and Michael VIII Palaeologus as co-emperors. Arsenius retired to a monastery when Michael extruded John from authority, but he was persuaded to return to office after Constantinople had been liberated from the Latins in 1261. When Michael banished and blinded John, Arsenius excommunicated the Emperor and was consequently deposed by him in 1265. He was exiled to Proconnesus, where he wrote a testament that has served as an important source of contemporary history.

After Arsenius' deposition, the empire was split into two factions known as the Arsenites (followers of Arsenius) and the Josephists (followers of Joseph, Arsenius' second successor). The Arsenites fanatically opposed Michael's pro-Latin policy, which culminated at the second Council of Lyon in 1274, when papal supremacy over the Greek Church was accepted by Michael's legates. The Arsenite schism continued after the council had been rejected by the Byzantines and officially abandoned as imperial policy, and it did not end until 1310, when the body of Arsenius was buried by Patriarch Niphon in Hagia Sophia, the primatial church in Constantinople.