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 Nicaea 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.