Marcionism

Marcionism is a sect founded in A.D. 144 at Rome by Marcion and continuing in the West for 300 years, but in the East some centuries longer, especially outside the Byzantine Empire. They rejected the writings of the Old Testament and taught that Christ was not the Son of the God of the Jews, but the Son of the good God, who was different from the God of the Ancient Covenant. They anticipated the more consistent dualism of Manichaeism and were finally absorbed by it. As they arose in the very infancy of Christianity and adopted from the beginning a strong ecclesiastical organization, parallel to that of the Catholic Church, they are considered by the Catholic Church to have been the most dangerous foe Christianity has ever known.

Largely copied from the Catholic Encyclopedia (1908)