So Mark decided to create a “story of Jesus” and intended it to serve as scripture, but what will have been the source for the overall outline of that story? Could he have created it from scratch, devising his own plan for fitting numerous short vignettes about Jesus into a cohesive whole? I am convinced that he in fact utilized a story outline that had already been known among the Gentile churches.
(Paul Nadim Tarazi, Paul and Mark, p. 126, my bold)
Marcion accepted enthusiastically this popular, pagan idea of Christ’s death; its simplicity appealed to him. It was looked upon as an accomplished event, and was not hampered with a baggage of visions, interpretations, gnoses, and what not. It was eminently readable and, read aloud in the churches, would arouse more fervid faith than the most ebullient prophecy. The manifestation of God, extraneous to the world, could be told in the form of a brief tale of Jesus on earth, concluding with the death on the cross, the sacrifice for the salvation of mankind, which St. Paul considered the essential and lasting act of Christ.
Paul-Louis Couchoud, Creation of Christ, t. I, p. 134-135, my bold)