Key councils: Rome (382 AD, under Pope Damasus), Hippo (393 AD), Carthage (397 AD, 419 AD). Key figures: Athanasius of Alexandria (Festal Letter 39, 367 AD — first exact list of 27 NT books), Jerome, Augustine.

The New Testament canon was not decided at Nicaea (325 AD) — a persistent myth. The canonical question was not on Nicaea’s agenda; Nicaea dealt with Arianism and the nature of Christ. The 27-book NT canon is first listed in exact form by Athanasius in his Easter letter of 367 AD, and formally ratified at the councils of Hippo (393) and Carthage (397, 419).

The path there is a 350-year process of selection from a much larger field: the Didache, Shepherd of Hermas, Epistle of Barnabas, 1 Clement, Gospel of Thomas, Gospel of Peter, and many others circulated with quasi-canonical authority in various communities. The selection criteria debated included: apostolic authorship or connection, orthodox theological content, and catholicity (wide use across churches). Disputed books (antilegomena) included Hebrews, James, 2 Peter, 2–3 John, Jude, and Revelation — all eventually included. The Eastern and Western churches diverged on some of these into the 5th century. The Catholic deuterocanon includes 7 OT books not in the Protestant canon — a dispute that explodes at the Reformation and is formally settled (for Catholics) at Trent (1546).