The Great Schism resulted largely from a disagreement over

It started when the patriarch of Constantinople and partiarch of Rome excommunicated each other.

The Great Schism resulted from a disagreement over the Eastern Christian churches and the Western churches after many political and religious conflicts between their leaders; Pope Leo IX from Western churches and the Patriarch of Constantinople, Michael Cerularius from Eastern churches. Their disagreements had as consequence mutual excommunications (lifted in 1964) and a permanent Schism between both churches that gave birth to the Orthodox Church.