In early Christian theology, the doctrine that God the Father and God the Son have different essences or substances, contrary to orthodox Christian belief.
From Greek hetero- (different) and ousia (essence/substance), literally 'different essence,' a theological term from the 4th-century Christian debates about the nature of the Trinity.
This was literally a word that caused religious wars: the Arian controversy of the 300s AD was fought over whether Father and Son had the same essence (homoiousia) or different essences (heteroousia)—spelled so similarly yet theologically worlds apart!
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