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Great Schism
The division of the Church into Eastern Orthodoxy and Roman Catholicism — formalized in 1054 but centuries in the making.
The Great Schism of 1054 formalized the separation triggered by the mutual excommunication of Cardinal Humbert and Patriarch Michael Cerularius. The deeper causes: the Filioque, differing understandings of papal primacy, liturgical divergences, and political separation. The mutual excommunications were lifted in 1964, but full communion has not been restored.