Matthew, the Perverted Gospel of Galatians

Christ and Peter walk on water, 
Fresovo taken from Matthew 14:28-31 
Dura-Europos (mid 3rd century?)

Most scholars when they hear Marcionite priority assume this means that the first Gospel was Marcion's and all other are dependent upon it. But Paul is still regarded generally as before the Gospels. Both assumptions are wrong. Marcionite priority merely means the first publicly circulating Gospel was the Marcionite. The letters of Paul are in earlier form in Marcion's collection, but their relationship to the Gospel is less clear, as we shall see with Galatians. [1]

The entire purpose of the first Gospel was to spread the word of Christ throughout the Roman Empire. And not surprising it was sectarian, the Marcionite Gospel. Other main sect, the proto-Orthodox, quickly found themselves at a disadvantage in this new game of evangelism, where itinerant teachers (Apostles).

Galatians letter presents a picture if a divided movement. One where Paul finds his authority challenged, with rivals dismiss his teachings, and present a completely different version of Christ. This letter, even more clearly in Marcionite form, presents a scenario where the first teachings of Paul have been overturned. What then is this new Christianity Paul confronts?  How is it that this new and different message has become such a threat that it required such strong a response?