Supported the missionary work of Saint Cyril and Saint Methodius, and approved the use of Slavonic language in the liturgy.
Strongly opposed the work of SaintIgnatius of Constantinople in Bulgaria.
When Ignatius died, John recognized Photius as patriarch and called the council (879-880) that briefly reconciled differences between East and West.
He did much to root out corruption in the church in Rome, excommunicated corrupt nobility, and was one of the strongest popes of the 9th century, though he was forced to bribe Saracens to keep them out of Rome.
John was deeply involved in imperial politics.
He crowned Charles the Bald as emperor, and excommunicated the future Pope Formosus for opposing him.
When Charles lost power, John favored Charles the Fat, who became Emperor Charles III.
The first pope to be killed in office for non-religious reasons.