Marcionites were the followers of Marcion. If these types of Christians had become the dominant type, they would have canonized a different set of New Testament texts to those that the Pauline Christians did. As the Pauline Christians proved themselves the more aggressive, violent and powerful, the Marcionites were eradicated by them.
Mainstream Pauline Christianity has a problem with The Bible. The Old Testament is completely different to the New Testament. The God of the Old Testament is murderous, demanding, jealous, vocal, interfering and debates with its people. The God of the New Testament, however, appears largely removed, is highly spiritual, peaceful and reasoned. Hundreds of Christian books have debated what the causes are of this difference.
Christian Marcionites resolved the difference thusly: They believed in 2 Gods. The God of the Old Testament was actually a different God to the (better) God of the New Testament.
“As we have seen, Paul claimed that a person is made right with God by faith in Christ, not by doing the works of the Law. This distinction became fundamental to Marcion, and he made it absolute. The gospel is the good news of deliverance; it involves love, mercy, grace, forgiveness, reconciliation, redemption and life. The Law, however, is the bad news that makes the gospel necessary in the first place; it involves harsh commandments, guilt, judgement, enmity, punishment, and death. [...]
How could the same God be responsible for both? Or put in other terms: How could the wrathful, vengeful God of the Jews be the loving, merciful God of Jesus? Marcion maintained that these attributes could not belong to one God. [...]
The God of the Old Testament [...] was not evil, but he was rigorously just. He had laws and inflicted penalties on those who did not keep them. But this necessarily made him a wrathful God, since no one kept all of his laws perfectly. Everyone had to pay the price for their transgressions, and the penalty for transgression was death.”
This situation, with the God of the Old Testament being involved with the world and laying down laws, would change when the other God, who had so far not been involved, took pity on mankind and decided to try to save us.
“The God of Jesus came into this world in order to save people from the vengeful God of the Jews. [...] He paid the penalty for other people's sins, to save them from the just wrath of the Old Testament God.”
Early Christians had many opinions on the nature of Christ. Marcion's was that Christ was an appearance of a Human (a belief called docetism), in order to trick the Old Testament God. Only another God could obey the OT laws perfectly, and only a person who followed them perfectly and then sacrificed himself could end the rule of the Old God. So the God of Jesus, the NT God, appeared on Earth pretending to be a Human. The Good News was spread that Jesus had indeed come and made himself the perfect sacrifice. The power of the OT God was reduced, and now began the age of pisces, the NT God.
Paul wrote that Jesus did indeed only come 'in the likeness of flesh' (Rom. 8:3), and by having faith in Jesus you escaped the wrath of the OT God. Jesus said that a tree is known by its fruit (see Luke 6:43-44): "Good trees do not produce rotten fruit, and rotten trees do not produce good fruit". Marcion realized that this applies to the OT: The Old Testament God, who says that he "creates evil" (Amos 3:6), cannot have produced the New Testament. To say otherwise is to ignore the words of Jesus and Paul.
In Pauline Christianity, the reasons for the crucifixion of Jesus are a little vague and confusing. If the Old Testament God was the same as the New, and it wanted to save more people, then it could merely have chosen to do so. There was no need for an 'ultimate sacrifice'. Being the supreme God, it could save whoever it wants, whether or not Jesus came to Earth and died on the cross. Marcionite Christianity did not have this problem; there was a clear reason why Jesus died: To trick the OT God out of its power.
Marcionism was wide and successful, for hundreds of years some places knew only of Marcionite Christianity. Pauline Christianity, although aggressive and with its resources and power, could not seem to oppress it. It did not have the logical problems of saying that the harsh God of the Old Testament had somehow 'changed', and it didn't have the problems of what to do with all the Old Testament laws and regulations: Other Christians such as the Ebionites still obeyed all these laws. Marcionism portrayed Jesus as a good God come to save humanity, and as such, was a powerful and influential Christian movement.
“Marcion put together a canon of Scripture, that is, a collection of books that he considered to be sacred authorities. Marcion, in fact, is widely thought to have been the first Christian to have done so [...], long before the New Testament that we know was established.”
Ehrman notes that some scholars think that Marcion's example was copied by the continually growing Pauline Christians. Aside from copying Marcionism, Pauline Christians continually attacked it, devoting resources and money to do so.
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. Tertullian produced five volumes attacking Marcionism and distributed them throughout the Roman Empire. The honest intellectual and rational approach of Marcion to the Old Testament and the saving grace of Jesus were lost, burnt and oppressed by the more violent and aggressive Pauline Christians. It is ironic that in the name of 'good works', Pauline Christians murdered and tortured those who believed differently to themselves... if it is true, as Jesus says and as Marcion pointed out, that good trees do not produce rotten fruit, then have we ended up with a rotten tree grown from a rotten fruit, instead of the real Christianity as espoused by Jesus? 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.