Against Heresies (St. Irenaeus)

The most important early Christian anti-Gnostic work (c. 180 AD). Exposes Gnostic systems and refutes them using Scripture, Tradition, and apostolic succession. Develops the theology of 'recapitulation': Christ sums up all of humanity in Himself, reversing Adam's fall. Foundational for Orthodox understanding of Tradition and ecclesiology.