Marcionites had some great theology, and some great solution to some otherwise-unsolved problems within Christianity. It's a shame that the more sensible ones are frequently the less aggressive... as they were of course wiped out by their less compassionate Pauline Christian brethren.
Marcionites believed that the God of the Old Testament was a different God to the new testament. Their reasoning is sensible, and their collection of Christian books into a canon was the first ever collection.
"At one point, the early Christian writings that were collected by Marcion, along with his own writings, were all destroyed. A domineering early Catholic Church, the Pauline Christians, committed themselves to a long-term campaign against these early Christians. [...] If it is Christian duty to 'turn the other cheek', 'resist not evil', 'love your enemies' and 'love your neighbours as yourself', then it is clear that the Pauline Christians, who eliminated Marcionism and got to choose the books of the Bible, were not the true Christians. "