There has been a long-standing, minor debate about whether Luke copied Marcion or vice versa. The evidence that Marcion copied Luke is so strong that there should be no real debate.

First of all, there is compelling evidence that Luke used Mark's Gospel as his primary source for the life of Jesus. Whenever Luke repeats a passage from Mark, the Greek text is almost identical. Luke's Gospel is famous for the "missing block" of material from Mark, that results in the puzzling verse at Luke 9:18 "And it came to pass, as he was alone praying, his disciples were with him ...", which can be explained by looking at Mark 6:47 and Mark 8:27. One of the authors must have copied Mark, and it is unlikely to have been Marcion. Luke's Gospel and Luke's Acts of the Apostles both appear to contain background material sourced from Josephus. Since there is no dispute that Luke wrote Acts, this makes it highly probable that he did not copy the Gospel from Marcion.

Marcion's Gospel is similar to Luke's Gospel, but omits references to the Old Testament, which Marcion rejected. It is not plausible that Marcion wrote a gospel without references to the Old Testament and then Luke copied Marcion and skilfully put appropriate Old Testament into his version.

Marcion wrote his gospel after 140 CE. So if Luke copied Marcion, he would have written his gospel after about 150 CE, which is too late to be plausible.

On the other handMarcion as pre-dating LukeCharles B. Waite was one of the first to propose in History of the Christian Religion to the Year Two-Hundred in 1881 that Marcion's Gospel may have preceded Luke's Gospel. John Knox (not the same as the Scottish reformer John Knox) in Marcion and the New Testament also defends this hypothesis. Some recent scholars have agreed. In this case, Luke's gospel was not finished. There are two possibilities: Either Marcion and Luke both based their gospels on an earlier, common source (such as Matthew and Mark in the Augustinian hypothesis), or the Gospel of Luke was based on Marcion's gospel.

For those who wish to do some review, use the link below to the Gospel of Marcion in Wikipedia. An introduction can be found there, and links to other sources where the full text is posted are available in the footnotes.