Meanwhile, the potential conversion of Khan Boris I of Bulgaria () to Christianity under Frankish influence may have caused Michael and Bardas to threaten to invade Bulgaria. Anyhow, by the 860s, Boris converted to Christianity of the Byzantine rite. Michael stood as sponsor, by proxy, for Boris at his baptism. Boris took the additional name of Michael at the ceremony. The Byzantines also allowed the Bulgarians to reclaim the contested border region of Zagora. In 866, Boris appeared to have second thoughts and sought patronage from the Franks and Pope Nicholas I, requesting a Latin archbishop. He corresponded with Nicholas, who responded to him by expressing approval of Bulgar customs and severely criticising those of Byzantium. Photios responded by enumerating western doctrinal and disciplinary deviations in an encyclical, deposing Nicholas and requesting Louis the German's aid in overthrowing him. Michael presided over a synod in 867 in which Photios and the three other eastern patriarchs excommunicated Pope Nicholas and condemned the Latin filioque clause concerning the procession of the Holy Spirit. The conflict over the patriarchal throne and supreme authority within the church was exacerbated by the success of the active missionary efforts launched by Photios. In the same year, Michael was assassinated and Basil reinstated Ignatios. Despite Ignatios' papal leanings in the Photian schism, he allowed Boris to procure a Byzantine-rite archbishop in 870, and the eastern patriarchs forcefully denied any Roman authority in Bulgaria. Byzantine cultural hegemony was secured, and relations between the two empires remained peaceful throughout Michael's reign. The conversion of the Bulgarians has been evaluated as one of the greatest cultural and political achievements of the Byzantine Empire.
Meanwhile, the potential conversion of Khan Boris I of Bulgaria () to Christianity under Frankish influence may have caused Michael and Bardas to threaten to invade Bulgaria. Anyhow, by the 860s, Boris converted to Christianity of the Byzantine rite. Michael stood as sponsor, by proxy, for Boris at his baptism. Boris took the additional name of Michael at the ceremony. The Byzantines also allowed the Bulgarians to reclaim the contested border region of Zagora. In 866, Boris appeared to have second thoughts and sought patronage from the Franks and Pope Nicholas I, requesting a Latin archbishop. He corresponded with Nicholas, who responded to him by expressing approval of Bulgar customs and severely criticising those of Byzantium. Photios responded by enumerating western doctrinal and disciplinary deviations in an encyclical, deposi?ng Nicholas and requesting Louis the German's aid in overthrowing him. Michael presided over a synod in 867 in which Photios and the three other eastern patriarchs excommunicated Pope Nicholas and condemned the Latin filioque clause concerning the procession of the Holy Spirit. The conf,lict over the patriarchal throne and supreme authority within the church was exacerbated by the success of the active missionary efforts launched by Photios. In the same year, Michael was assassinated and Basil reinstate!d Ignatios. Despite Ignatios' papal leanings in the Photian schism, he allowed Boris to procure a Byzantine-rite archbishop in 870, and the eastern patriarchs forcefully denied any Roman authority in Bulgaria. Byzantine cultural hegemony was secured, and relations between the two empires remained peaceful throughout Michael's reign. The conversion of the Bulgarians has been evaluated as one of the greate.st cultural and political achievements of the Byzantine Empire.