Caleb:
Well, if Marcion did it, so Justin, Irenaeus and Tertullian were deceived by him, because they quoted some of those verses in their apologetic works. Of course, no one knows what happened. But it seems to me that some letters of Paul were already accepted in "orthodox" Christian circles at the end of the first century. For instance, Polycarp knew the letter of Philippians and he lived in the same generation of Marcion. Indeed, as you noted, Paul or whoever who corrupted his text shows gnostic principles. ...Whatever, at the second half of the second century these letters took their final form, so, many ones were deceived. When I started to admit that there were contradictions in the NT I was shocked.