Sources: Ignatius of Antioch, Letters (~107 AD); 1 Clement (~96 AD); Didache (~100 AD); Polycarp of Smyrna, Letter to the Philippians (~110 AD); the Muratorian Canon (~170 AD).

Proto-Catholic Christianity (~100–200 AD) is the movement within early Christianity that developed the institutional structures that became Catholic and Orthodox Christianity: episcopal hierarchy, canon, creed, and apostolic succession. Against the Gnostics (who claimed special knowledge), Marcionites (who rejected the Old Testament), and Montanists (who claimed ongoing prophecy), the proto-Catholics developed a three-fold defense: (1) The Rule of Faith (regula fidei) — a brief summary of apostolic teaching that preceded and generated the formal creeds; (2) Apostolic succession — the bishop’s authority traced to the apostles guarantees authentic tradition; (3) The New Testament canon — a defined collection of apostolic writings alongside the Old Testament. Ignatius of Antioch’s letters are the earliest evidence for monepiscopacy (one bishop per city): “Where the bishop appears, let the congregation be present, just as where Jesus Christ is, there is the Catholic Church” (Smyrnaeans 8:2) — the earliest use of “Catholic Church.” Irenaeus of Lyon (~180 AD) is the great synthesizer who most fully articulates the proto-Catholic position against the Gnostics: his Against Heresies defines orthodoxy by reference to the apostolic tradition preserved in the succession of bishops in the major churches (Rome, Antioch, Alexandria).