It is important to note that although Marcionism is related to Gnosticism, it is not considered a wholly Gnostic sect.
Primarily, Marcionism rejected the Old Testament and any account of it to be a departure from the truth. Marcionists believed that the god of the Old Testament was a wrathful, genocidal and jealous being. Because of this, the Old Testament god created the material world in error. Thus, the world is an earthly trap, a place of suffering.
The Old Testament God was the source of evil. Marcion wanted to teach of a lesser-known Christ – the God of the New Testament gospels. Marcion composed a work called the Antithesis. This work juxtaposed traditional Jewish scriptures with the Gospel of Luke and the Pauline Epistles. Marcion’s version of the Gospel of Luke is not the canonical version read today.
The Marcionites also held a different view of Jesus. He was not the Jewish Messiah; rather, Jesus was a spiritual being sent by the Monad to free humanity from the worldly trap of the demiurge. The Marcionites also held an interesting view of the Good God. To the Marcionites, God is the Stranger or Alien God. This God has no previous interactions with the world and is wholly unknown.