Marcion was a native of Sinope (modern Sinop, Turkey), in Pontus, Asia Minor. He was a wealthy shipowner. According to St. Hippolytus of Rome, he was the son of a bishop who excommunicated him on grounds of immorality. He eventually found his way to Rome (ca. 140) and became a major financial supporter of the Church there.
In the next few years after his arrival in Rome, he worked out his theological system and began to organize his followers into a separate community. He was excommunicated by the Church at Rome in 144. From then on, he apparently used Rome as a base of operations, devoting his gift for organization and considerable wealth to the propagation of his teachings and the establishment of compact communities throughout the Roman Empire, making converts of every age, rank and background.
A story told by Tertullian and St. Irenaeus of Lyons says that Marcion attempted to use his money to influence the Church to endorse his teaching, but was refused. His numerous critics throughout the Church include the aforementioned, along with St. Justin Martyr, St. Ephraim of Syria, Dionysius of Corinth, Theophilus of Antioch, Philip of Gortyna, St. Hippolytus and Rhodo in Rome, Bardesanes at Edessa, Clement of Alexandria, and Origen.
Marcion's teaching, known as Marcionism, was that Jesus revealed to the world a hitherto unknown god, who was different from the god of the Hebrew Bible. According to Marcion, the god of the Hebrew Bible was jealous, wrathful, and legalistic. The material world he created was defective, a place of suffering; the god who made such a world was the bungling or malicious demiurge. Jesus was not the Messiah promised by Judaism; that Messiah was to be a conqueror and a political leader. Rather, Jesus was sent by a god greater than the Creator. His role was to reveal the transcendent god of light and pure mind, different in character from the creator god of the Hebrew Bible. Jesus's god was free from passion and wrath, wholly benevolent; and Jesus was sent to lead believers out of subjection to the limited, wrathful creator god of the Old Testament.
Marcion produced the first Christian canon, or list of the books of the Bible that he considered authoritative. His list, however, was much smaller than that currently recognised as valid by most Christians: he included only the Gospel of Luke, the Acts of the Apostles, and ten of the epistles attributed to the Apostle Paul. (He omitted Paul's pastoral epistles, addressed to Timothy and Titus.) These books, according to Marcion, were the ones that contained the true teachings of St. Paul. He completely rejected the Old Testament, believing and teaching that it should not be part of the Christian Bible and was of no value to Christians.
Marcion's position is not identical to, but is closely related to, the various belief sets together called Gnosticism. In some sources, he is often reckoned among the Gnostics, but as the Oxford Dictionary of the Christian Church (3rd ed.) puts it, "it is clear that he would have had little sympathy with their mythological speculations" (p. 1034). Like the Gnostics, his Christology was Docetic.
His thinking, untenable to most Christians throughout history, shows the potential influence of Hellenistic philosophy on Christianity, and the moral critique of the Hebrew Bible from the ethics of Platonism. Marcion's proposed canon was a factor that led the orthodox Christian movement to formulate a canon of authoritative Scripture of its own, and which led to the current canon of the New Testament.
His writings have all been lost, but it is possible to reconstruct and deduce a large part of what he taught based on what other writers said concerning him, especially Tertullian. He was also known to have imposed a severe morality on his followers, some of whom suffered in the general persecutions of Christians.
The Oxford Dictionary of the Christian Church (3rd ed.), pp. 1033-34