The question in debate may be stated thus.

Did Marcion, as the Church writers say, really mutilate our so-called St.Luke (the name is not of importance, but we may use it as standing for our third Synoptic in its present shape)?
Or, is it not possible that the converse may be true, and that Marcion's Gospel was the original and ours an interpolated version?
The importance of this may, indeed, be exaggerated, because Marcion's Gospel is at any rate evidence for the existence at his date in a collected form of so much of the third Gospel (rather more than two-thirds) as he received.

Still the issue is not inconsiderable: for, upon the second hypothesis, if the editor of our present Gospel made use of that which was in the possession of Marcion, his date may be--though it does not follow that it certainly would be--thrown into the middle of the second century, or even beyond, if the other external evidence would permit; whereas, upon the first hypothesis, the Synoptic Gospel would be proved to be current as early as 140 A.D.; and there will be room for considerations which may tend to date it much earlier.

There will still be the third possibility that Marcion's Gospel may be altogether independent of our present Synoptic, and that it may represent a parallel recension of the evangelical tradition.

This would leave the date of the canonical Gospel undetermined.
It is a fact worth noting that the controversy, at least in its later and more important stages, had been fought, and, to all appearance, fought out, within the Tuebingen school itself.
Olshausen and Hahn, the two orthodox critics who were most prominently engaged in it, after a time retired and left the field entirely to the Tuebingen writers.
The earlier critics who impugned the traditional view appear to have leaned rather to the theory that Marcion's Gospel and the canonical Luke are, more or less, independent offshoots from the common ground-stock of the evangelical narratives.