Trinitarian theology developed gradually from the early Church's doxological and liturgical practice against both Modalism (which collapsed the distinctions between Father, Son, and Spirit) and Subordinationism (which compromised the Son's full divinity). Tertullian coined the Latin term trinitas and the substance/person distinction against Modalism. Athanasius anchored the Nicene settlement against Arianism, and the Cappadocians — Gregory of Nyssa in particular — pressed the case for the Spirit's full divinity against the Pneumatomachians. Augustine's On the Holy Trinity was the West's most systematic treatment, exploring the terminological differences between Greek (hypostaseis) and Latin (personae) while affirming the same underlying reality.
"He maintains that there is one only Lord, the Almighty Creator of the world, in order that out of this doctrine of the unity he may fabricate a heresy. He says that the Father Himself came down into the Virgin, was Himself born of her, Himself suffered, indeed was Himself Jesus Christ" — Tertullian's description of Modalism and the error that drove his formulation of the trinitas vocabulary.
Augustine examines whether the Greeks' three hypostaseis and the Latins' three personae name the same reality — probing the limits of Trinitarian language and concluding that both traditions confess one substance in three, though their terminological conventions differ.
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