Marcionism was an early Christian dualistic belief system originating with the teachings of Marcion of Sinope in Rome around 144. Marcion was an early Christian theologian, evangelist, and an important figure in early Christianity. He was the son of a bishop of Sinope in Pontus. About the middle of the 2nd century (140–155) he traveled to Rome, where he joined the Syrian gnostic Cerdo.

Marcion preached that the benevolent God of the Gospel who sent Jesus into the world as the savior was the true Supreme Being, different and opposed to the malevolent deity, the Demiurge or creator deity, identified with Yahweh in the Hebrew Bible. He considered himself a follower of Paul the Apostle, whom he believed to have been the only true apostle of Jesus.

Marcion's canon, possibly the first Christian biblical canon ever compiled, consisted of eleven books: the Gospel of Marcion, which was a shorter version of the Gospel of Luke, and ten Pauline epistles. Marcion's canon rejected the entire Old Testament, along with all other epistles and gospels of what would become the 27-book canon of the New Testament, which during his life had yet to be compiled.

Marcionism was denounced by its opponents as heresy and written against by the Church Fathers – notably by Tertullian in his five-book treatise Adversus Marcionem (Against Marcion), in about 208. Marcion's writings are lost, though they were widely read and numerous manuscripts must have existed. Even so, many scholars say it is possible to reconstruct and deduce a large part of ancient Marcionism through what later critics, especially Tertullian, said concerning Marcion.

According to Tertullian and other writers of early proto-orthodox Christianity, the movement known as Marcionism began with the teachings and excommunication of Marcion around 144. Marcion was reportedly a wealthy shipowner, the son of a bishop of Sinope of Pontus, Anatolia. He arrived in Rome c. 140, soon after the Bar Kokhba revolt. The organization continued for some centuries later outside the Byzantine Empire in areas which later would be dominated by Manichaeism, particularly as the Sasanian Empire.^{[citation needed]}

By the reign of emperor Commodus (180–192), Marcionism was divided into various opinions with various leaders; among whom was Apelles, whom Rhodo describes as "priding himself on his manner of life and his age, acknowledges one principle, but says that the prophecies are from an opposing principle, being led to this view by the responses of a maiden by name Philumene, who was possessed by a demon". However, "others, among whom were Potitus and Basilicus, held to two principles, as did Marcion himself. Others consider that there are not only two, but three natures. Of these, Syneros was the leader and chief.”

In the early 3rd century, a splinter group of Marcionites was established by Prepon the Syrian, who claimed the existence of an intermediate spiritual entity between the good and evil gods.

The premise of Marcionism is that the ministry of Jesus is incompatible with the actions of God in the Old Testament. Focusing on the Pauline traditions, Marcion felt that all other conceptions of the Gospel, and especially any association with Israelite religion, was 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, death and life, as the essence of religious truth. He ascribed these aspects and characteristics as 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 who is only love and mercy.