Marcionism is the dualist belief system that originated in Rome from the teachings of Marcion of Sinope around the year 144. Marcion affirmed Jesus Christ as the savior sent by God and Paul as his chief apostle, but he rejected the Hebrew Bible and Yahweh. Marcionism anticipated the more consistent dualism of Manichaeism.
As Marcionism arose in the very beginning of the Christian era and from the very start had adopted a strong ecclesiastical organization that paralleled that of the Orthodox Christian Church, the movement was a dangerous foe of Christianity. While Marcionism has been associated with Gnosticism, Marcion looked to a form of Christianity that had no association with Judaism. Marcion’s vision seemed centered around the texts that were being used by Christians for a new testament, an approach that led the Orthodox on a path of defining the New Testament.
Early on, Marcionism was denounced by its opponents as heresy. These opponents also wrote against it, notably by Tertullian in a five-book treatise titled Adversus Marcionem that was written about 208. The criticisms against Marcionism, thus, predate the authority, claimed by the First Council of Nicea in 325, to declare what is heretical against the Church. Marcion's writings are lost, though they were widely read and numerous manuscripts must have existed. Even so, many scholars (including Henry Wace) claim it is possible to reconstruct and deduce a large part of ancient Marcionism through what later critics, especially Tertullian, said concerning Marcion.
Marcion declared that Christianity was distinct from and in opposition to Judaism. He rejected entirely the Hebrew Bible and declared that the God of the Hebrew Bible was a lesser demiurge, who had created the earth, but was (de facto) the source of evil.
The premise of Marcionism is that many of the teachings of Christ are incompatible with the god of the Jewish religion. Focusing on the Pauline traditions of the Gospel, Marcion felt that all other concepts of the Gospel, and especially any association with the Old Testament religion, were opposed to, and a backsliding from the truth. He further regarded the arguments of Paul regarding law and gospel, wrath and grace, works and faith, flesh and spirit, sin and righteousness, and death and life as the essence of religious truth. He ascribed these aspects and characteristics to two principles, the righteous and wrathful god of the Old Testament, who is at the same time identical with the creator of the world, and a second God of the Gospel, quite unknown before Christ, who is only love and mercy. Marcion gathered scriptures from the Jewish tradition, and juxtaposed these against the sayings and teachings from the Gospel of Luke, the Epistles of Paul (but not the Pastoral Epistles or the Epistle to the Hebrews), and the added Marcionite Epistle to the Laodiceans, in a work entitled the ‘‘Antithesis’‘. Marcion’s version of Luke did not resemble the version that is now regarded as canonical. It not only lacked all prophecies of Christ's coming and had differences with the now canonical version, as well as other serious theological implications. In bringing together these texts, Marcion redacted what is perhaps the first attempt at a New Testament canon on record, which he called the Apostolikon, which reflected his belief in writings associated with the apostle Paul and Jesus.
After Marcion’s excommunication, elements of his movement continued in the Mediterranean west for about 300 years, and in the east for some centuries more, principally in areas outside of the Eastern Roman empire that followed Manichaeism.