On this latter point, see Irenaeus, Haereses 1.27.2; 3.12.12; Tertullian, Marc. 4.6.2; Epiphanius, Pan. 42.9.1; 42.11.3–6. For a recent reconstruction of Marcion's Gospel, see Roth, who also provides a history of research (pp. 7–45).
The debate in NTS gathers three scholars who have recently published substantial volumes on Marcion: Matthias Klinghardt (his book here, 2015) and Jason BeDuhn (his book here, 2013) both argue against the charge leveled by Marcion's patristic opponents that Marcion redacted a form of Luke's Gospel, asserting to the contrary that Marcion's Gospel preceded both Marcion and Luke and was in fact the earliest Gospel written. [This is different from the view of Markus Vinzent, who thinks that Marcion himself actually wrote the earliest Gospel; for a critical interaction with Vinzent's book, see here.] The third participant in this exchange is Judith Lieu (her book here), who takes a somewhat more traditional line.
The rest of this post offers some summary and choice quotations from the three essays.
In the present article, Klinghardt summarizes his work, arguing that
in almost every single instance the direction of the editorial process runs from the Marcionite Gospel to Luke. Some passages - such as the beginning of the gospel or the account of the Last Supper - confirm this editorial direction beyond any doubt. True, there are indeed a few examples where the editorial process could run in either direction, but none of these examples requires, or even suggests, a reversal of the Marcionite priority. (319)How do the other Gospels - especially Matthew and Mark - fit into the picture?
The most obvious consequence of the priority of the Marcionite Gospel over Luke relates to the Synoptic Problem: when taking this 'pre-Lukan' gospel into account, the model of the inter-gospel relations changes profoundly. Most remarkably, this model disposes of the need for 'Q': the Two-Source Theory becomes entirely redundant, and the other models in discussion - such as the Farrer-Goulder-Goodacre hypothesis or the Neo-Griesbach Theory - are irrelevant. (320)
Klinghardt suggests that the Marcionite Gospel is first, on which Mark depended. Matthew used both Mark and the Marcionite Gospel. John used the previous three, and Luke used all the previous four.
"The search for the 'historical Jesus', therefore, becomes a completely different, if not an impossible, task" (321).
"although it is not impossible that a gospel existed before the middle of the second century, there is simply not even the slightest shred of evidence for any written gospel prior to that time" (322).
Klinghardt notes that many of the readings attributed to Marcion's Gospel show up as variant readings in the textual tradition of Luke, leading him to conclude that "this gospel was not the arbitrary product of a mean-spirited heretic but, quite simply and obviously, an older text utilised by many, including Marcion himself. And that text was, quite simply and obviously, edited by Luke" (322).
Once we ... objectively examine the texts of the two gospels, it becomes immediately clear that Marcion's Gospel cannot be an ideologically motivated redaction of Luke, for the simple reason that the two gospels are practically identical in ideology. (324)What separates BeDuhn from Klinghardt is, according to BeDuhn (p. 326), that Klinghardt sees Luke as an anti-Marcionite redaction of the Marcionite Gospel, and BeDuhn sees Luke as a Marcionite-neutral redaction of the Marcionite Gospel, which perhaps took place prior to Marcion. He says: "...it could even be suggested that Luke is a second edition of Marcion's Gospel by the same author." BeDuhn also thinks that the Two-Source hypothesis may be correct, once Luke is replaced in the equation with the Marcionite Gospel and the reconstruction of Q proceeds along these new lines.
BeDuhn takes up his last couple of pages addressing the issue of whether Marcion's scriptural collection provided the impetus for the non-Marcionite church to develop a canon of scripture. Whereas BeDuhn doesn't provide the answer here, he does (I seem to recall) argue in favor of this view in his book on the subject. But in this article he offers an interesting discussion of the divergent concepts of the collection of books promoted by Marcion and his opponents. He does seem to assume that Marcion had a clearly defined canon.
In this article she argues that our complete ignorance of Marcion and his Gospel from any source other than his opponents should make us cautious.
It is, therefore, misleading to suppose that Marcion's Gospel has survived and is available for comparative analysis, as one might with the canonical gospels. Although attempts to reconstruct Marcion's Gospel multiply, claims to achieve any precision must be treated with considerable scepticism. (330)[Elsewhere she judges rather positively Roth's work.]
Lieu then runs through some difficult obstacles, such as the textual transmission of the writings of our prime sources for Marcion - Tertullian and Epiphanius - as well as their accuracy in quoting Marcion and their selectivity, etc.
She appeals to the continuing influence of oral tradition on the written Gospels (331–32, an idea to which Klinghardt had objected, 321).
The editorial practices conventionally identified through redaction criticism are part of a much wider continuous activity encompassing both Marcion and Luke and different textual trajectories. On this model, the hypothesis that Marcion received, and probably edited, a predecessor of canonical Luke seems most likely. (332)As for Marcion's canon:
it is not at all evident that his gospel and 'apostolikon' formed a single corpus any more than they do in Irenaeus; their status in relation to each other remains problematic, as too does the so-called 'Antitheses', a work to which only Tertullian explicitly attests. At the same time the 'Jewish' scriptures continued to be necessary for his system, inasmuch as they demonstrated the multiple deficiencies of the Creator / Demiurge. It is anachronistic to speak of 'Marcion's Bible' or 'canon'. (332)