Its story
Marcion, a shipowner from Sinope on the Black Sea, came to Rome around 140 and made a large donation to the local church. Around 144 he was excommunicated (his money was returned) and founded his own network of churches, with bishops and liturgy, which spread quickly around the Mediterranean and the East.
His starting point was the contrast between Law and Gospel. In his Antitheses he set Old Testament texts beside sayings of Jesus to show that Israel's God—creator, just, lawgiver—could not be the merciful Father proclaimed by Christ. He therefore rejected the Jewish Scriptures and proposed a two-part canon: a Gospel (a form of Luke without certain passages) and an Apostolikon of ten Pauline letters. His Christology had docetic features: Jesus was not born of a woman.
The response was broad. Justin Martyr mentions him as an active teacher; Irenaeus reports that Polycarp, meeting him, called him 'the firstborn of Satan' (Against Heresies III.3.4); Tertullian devoted the five books of Against Marcion (c. 207–208) to him, our main source, defending the unity of God and of the two Testaments. Marcion's own writings are lost and are reconstructed from his critics.
Many historians think Marcion's canon, the first known, pushed the wider Church to define its own canon of apostolic writings, though they debate how much. Marcionite churches survived, especially in Syria, into the 5th century or later. The temptation to set the 'God of the Old Testament' against the 'God of Jesus' recurs periodically, which is why Marcion remains a reference point in modern theology (for example in Adolf von Harnack).