He supported Methodius of Thessalonica in his mission to the Slavs, defended him against the Carolingian rulers and Bavarian clergy, and authorized the translation of the Bible into Slavonic.

Unbeknownst to Rome, Methodius was imprisoned in 870 by the Carolingian King Louis the German and Bavarian bishops, who objected to his use of the Slavonic language in the liturgy and his encroachment on their jurisdiction in Moravia.

[3] After the raids against Campania and the Sabine Hills, Pope John asked for military aid from Emperor Charles the Bald and later Count Boso of Provence.

However, his efforts proved unsuccessful,[9] partly because Christian leaders viewed his calls for unity as an excuse to assert papal authority in southern Italy.

[8] In 876, John VIII traveled throughout Campania in an effort to form an alliance among the cities of Salerno, Capua, Naples, Gaeta and Amalfi against Muslim raids.

Charles again crossed the Alps, but this expedition was received with little enthusiasm by the nobles, and even by his regent in Lombardy, Boso, and they refused to join his army.

[14] However, modern scholarship particularly influenced by Catholic scholar Francis Dvornik has demonstrated this to be a Latin myth, as Photius died in visible perfect communion within the Roman Church.

[15] John was anxious that the Duchy of Croatia would follow in the steps of Bulgaria, which had recently accepted the spiritual authority of Constantinople rather than that of Rome.

[18] Without the protection of powerful magnates or the Carolingian emperor, the papacy after John VIII's reign became increasingly subject to the machinations and greedy ambition of the rival clans of the local nobility.