About the doctrine of the Trinity: this finds abundant attestation in the most ancient Church Fathers. A superb resource is Jurgens' Faith of the Early Fathers (three vols., about $45 from Eighth Day Books), which contains 1046 theological assertions with citations to passages from the Fathers, excerpted in the three volumes, that make this assertion. Thus for the doctrine "That God is triune is clear in the Tradition of the Church even in the pre-Nicene era," Jurgens offers no fewer than 39 passages, drawn from Clement of Rome, Ignatius of Antioch, "The Martyrdom of Polycarp," Dionysius Bishop of Corinth, Aristides of Athens, Justin Martyr, Athenagoras of Athens, Theophiilus of Antioch, that wonderful writer Irenaeus of Lyons, Tertullian, Hippolytus of Rome, Clement of Alexandira, Origen, Cornelius of Rome, Cyprian of Carthage, "Pope" Dionysius, Gregory Thaumaturgus, Aphraates the Persian Sage, and Athanasius. Incidentally, one fascinating thing about this set is how the chief orthodox doctrines of God, etc., find abundant early attestation, while matters such as the bodily assumption of the Lord's mother into heaven is first attested in Gregory of Tours (about AD 590).
Overview on the doctrine of the Trinity
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