It has been noted long before me that Marcion comes across looking like a predecessor of Martin Luther. In other words, the Catholic Church was pre-existent and Marcion had a new interpretation of Christ's message which essentially came from his own imagination.
I don't think that this is necessarily historically accurate. Marcionitism represents our first systematic theology. The Catholic Church seems by contrast more a reaction against Marcionitism than it does a preservation of something original. But I don't wish to get too deeply involved in these issues.
I would rather note that in spite of a number of unfortunate prejudices that come with being a German Protestant writing about a figure like Marcion, known for his rejection of the Jewish religion, von Harnack is informed enough and intelligent enough to reference other ancient Marcionite witnesses which contradict his assumptions about the religion. As such, in spite of the general incorrectness of his understanding about who and what Marcionitism was, von Harnack's work is so invaluable.
It might be useful for us to cite the most important 'fork in the road' in the study of Marcionitism and where von Harnack can be demonstrated to have taken the wrong path. It comes up with regards to the consistent report in the Patristic literature that the Marcionites possessed a gospel they claimed was written by the apostle who wrote the Apostolikon, the man we call 'St. Paul.'
After assembling an unbelievable amount of information about the Marcionites and their gospel, von Harnack had the difficulty of deciding which sources to give more weight than others. Since von Harnack believed in and accepted the surviving canon, von Harnack essentially accepted the reports of the Church Fathers that Marcion tampered with the Gospel of Luke. Yet von Harnack is the first to admit in order to explain what the Marcionite gospel was he had to ignore a great number of things said by the same Patristic writers.
Here is the original passage in Das Evangelium von frenden Gott. I have provided only a rough English translation (I simply don't have the time to do a better job). Nevertheless I think the reader will find it quite useful:
Unzweideutig sagt Paulus, daß er ein von Christus selbst direkt berufener Apostel, daß sein Evangelium nicht durch menschliche Vermittlung an ihn gekommen sei, daß er es vielmehr durch Offenbarung erhalten habe und zwar durch eine Entrückung in den dritten Himmel, d. h. in einen Himmel, der hoch über dem Weltenhimmel liegt. Hieraus schloß Marcion, daß Paulus als der Apostel von Christus berufen worden sei, um der falschen Predigt entgegenzuwirken, und ferner, daß ein Evangelium vorhanden sein müsse, das von keinem Menschen geschrieben, sondern direkt von Christus dargereicht sei - wie, darüber scheint sich Marcion keine deutliche Vorstellung gemacht zu haben. Die Schüler haben bald an Christus selbst als Verfasser gedacht, bald an Paulus (Adamant., Dial. I, 8; II, l3f.; Carmen adv. Mare. II, 29); aber Tert. berichtet nur IV, 2: "M. evangelio suo nullum adscribit auctorem." Bemerkenswert ist hier vor allem,daß Marcion es für selbstverständlich gehalten haben muß, (indem er gewisse Äußerungen des Paulus so deutete), daß Christus für ein authentisches geschrieben es Evangelium gesorgt hat - so verlassen war er von aller geschichtlichen Kunde und so gewaltsam machte er selbst Geschichte. (p. 39)
Paul says unequivocally that he himself was called apostle directly from Christ, that his gospel did not come to him by human agency, that he had rather receive it by revelation and through a rapture to the third heaven, that is, in a heaven that was high above the heavenly world. Hence Marcion concluded that Paul had been called as the apostle of Christ, to counter the false preaching, and further that a gospel must have been given there, not written by anyone, but given directly by Christ - how it seems to have made, Marcion had no clear idea. His students even soon thought of Christ as the author, later given to St. Paul (Adamant., Dial I, 8, II, l3f;. Carmen adv Mare II, 29..). But Tertullian reported only IV, 2: "Marcion evangelio suo nullum adscribit auctorem (Marcion to his gospel attaches no author's name). Remarkable is, above all, that Marcion must have taken for granted, (by certain comments of Paul so indicated), that Christ has provided an authentic written gospel - he was thus forsaken by any historical character and so forcibly he made history for himself.
I am the furthest thing from a reliable translator of German and unfortunately this very important book has never been rendered into English. Nevertheless the underlying sense is clearly that von Harnack is very aware that a conflict exists between the way we want to interpret the Marcionite literature (i.e. that they said it was produced by 'Paul') and what the Marcionites themselves revealed about their religion.
The most likely formula then is not that the Marcionites held that 'Paul' was the name of the author of the gospel but rather that 'Christ' wrote it and that their Christ was not Jesus but Marcion himself. There are just too many references from the Church Fathers where it is obvious that Marcion (= the diminutive form of 'Mark') was the Christ of the tradition. Here is just a couple of examples of Tertullian's reference to this idea during the course of his discussion of Mark 13.6/Luke 21.8:
then those people will come, saying I am Christ. You,
No man is for himself both claimant and witness. Besides this, you have found it written that many will come and say, I am Christ. If there is one that makes a false claim to be Christ, much more can there be one who professes that he is an apostle of Christ. [5.1]
There are at least two dozen other references that we can bring forward which demonstrate that, long before the coming of Mani, Marcion was conceived as the apostle and paraclete of Jesus.
The difficulty for traditional scholarship is that it is apparently very difficult for believers to examine the writings of the early Fathers critically. When Polycarp began the habit of referencing Marcion as the 'antichrist' he was almost certainly reacting to the pre-existent cult of Mark the Christ (which still remains in early Coptic homiles).
As F F Bruce notes antichristos has a wider range of meaning than pseudochristos. It may mean "a substitute Christ," "a rival Christ" or "an opponent of Christ." It is impossible to escape the meaning of 'substitute' or 'rival Christ' in Polycarp's use of the terminology especially when we see that it was connected with a rejection of the idea that Jesus was the predicted messiah of the Jews. So we read in Polycarp:
Everyone who does not confess that Jesus Christ has come in the flesh is antichrist. And whosoever does not confess the testimony of the cross is of the devil; and whosoever perverts the oracles of the Lord to his own lusts, and says that there is neither resurrection nor a judgment, — he is the first-born of Satan.
Bruce explains the terminology as follows:
"the testimony of the cross" he probably means the witness which our Lord's suffering and death bore to his real manhood (cf. John 19:35; 1 John 5:8). And the reference to "Satan's firstborn" reminds us of Polycarp's retort to Marcion, several years later, when Marcion invited the aged bishop to recognize him: "I recognize— Satan's firstborn! Any heretic in Polycarp's eyes was an antichrist; but a heresiarch like Marcion was the Antichrist himself, the firstborn of Satan as Christ was the first-begotten of the Father. (A Mind For What Matters p. 182)
Yet the underlying assumption here again certainly is that Marcion was raised up as a substitute Christ by the Devil. We see this over and over again in the literature that depends on Polycarp's understanding:
Now, the more firmly the antichrist Marcion had seized this assumption, the more prepared was he, of course, to reject the bodily substance of Christ [AM 3.8]
the abortive Marcionites, whom the Apostle John designated as antichrists, when they denied that Christ was come in the flesh [ibid]
the devils put forward Marcion of Pontus, who is even now teaching men to deny that God is the maker of all things in heaven and on earth, and that the Christ predicted by the prophets is His Son, and preaches another god besides the Creator of all, and likewise another son. And this man many have believed, as if he alone knew the truth, and laugh at us, though they have no proof of what they say, but are carried away irrationally as lambs by a wolf, and become the prey of atheistical doctrines, and of devils. For they who are called devils attempt nothing else than to seduce men from God who made them. [Apology 58]
It is thus almost certain in my mind that when the Marcionites referenced 'the gospel of Christ' they already recognized Mark (dim. 'Marcion') as that figure rather than Jesus. All the evidence from the anti-Marcionites points to this unshakable conclusion.
And then there is the reference to the St. Mark as Christ concept in the surviving Homilies of St. Mark attributed to Severus of Al'Ashmunein, the Passio Petri Sancti, the Coptic Papacy and much more.
Why then don't scholars see these other possibilities? Why persist in wrongly understanding that the Marcionite identified their apostle, paraclete and christ as 'Paul'? I think the Letter to Theodore will finally change all of this once and for all ...