Sources: Tertullian, Against Marcion (5 books, ~207 AD — the longest surviving anti-heretical work); Irenaeus, Against Heresies 1.27, 3.12; Epiphanius, Panarion 42; Justin Martyr, Apology 1.26, 58.

Marcionism (~144 AD) is the most dangerous theological movement in 2nd century Christianity — dangerous not because it was obviously wrong but because it was internally coherent and addressed a real problem. Marcion of Sinope (~85–160 AD) came to Rome around 140 AD, made a large financial donation to the church, and proposed a radical theological revision: the God of the Old Testament (creator, lawgiver, vengeful, tribal) is a different God from the Father of Jesus Christ (merciful, forgiving, previously unknown). The Old Testament God is real but inferior — the demiurge. Jesus reveals a higher God who has no connection to Jewish history. Marcion’s canon: an edited Gospel of Luke (removing Jewish elements) plus 10 Pauline letters (excluding the Pastorals, which Marcion correctly identified as later). He was excommunicated ~144 AD; the church returned his donation. The theological legacy is enormous: (1) Marcion forced the orthodox church to articulate a coherent theology of the two Testaments’ unity — Irenaeus’s Against Heresies and Tertullian’s Against Marcion are fundamentally responses to Marcion; (2) Marcion’s canon provoked orthodox canon formation — the need to define which books were authoritative; (3) The “Marcionite temptation” — the impulse to dissolve the tension between the testaments by ditching one — recurs in every generation of Christian history and has never fully disappeared. Harnack called Marcion “the first Protestant.”