Filioque Controversy

This is the centuries-long East–West dispute over inserting "and the Son" into the Creed's article on the Spirit's procession, advanced in the West by Carolingian theologians and resisted in the East notably by Photios of Constantinople. Distinct from the doctrine itself, it names the controversy as a church-dividing event. The traditions differ: the West defended the addition while the East condemned altering the Creed, making this a defining issue in the estrangement of the churches.