On the Holy Trinity is a fifth-century theological treatise by Theodoret of Cyrus. Composed in the contentious aftermath of the great ecumenical councils, this systematic work in thirty-four passages defends Nicene Trinitarian orthodoxy against contemporary heresies, including Arianism, Eunomianism, and Sabellianism. Employing scriptural exegesis and logical argument, it affirms the co-equality, co-eternity, and distinct hypostases of the Father, Son, and Holy pneuma. Its core content comprises a detailed defence of the consubstantiality of the Trinity, polemics against the subordination of the Son and pneuma, and careful theological distinctions between ousia and person. The treatise solidified pro-Nicene theology from an Antiochene perspective, representing a key link between the Cappadocian Fathers and later doctrine and illustrating Theodoret's early role as a defender of ecclesiastical orthodoxy. The complete Greek text survives, preserved in critical editions and translated into several modern languages.
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