So if Irenaeus's caricature of a 'Jewish Christian' community who opposed Paul what do we make of the references in Paul's letters to 'Jews' who opposed him? I strongly suspect that these comments were references to actual Jews rather than 'Jewish Christians.' To be sure that presents a problem for Galatians chapter 2. But I have long suspected that this biographical material was a later interpolation. I don't think that the author of Against Marcion 4 is commenting on this material as if it were in the Marcionite canon. Rather I think - as I have consistently argued - that the author is arguing from his Galatians-first canon of Pauline writings rather than the Marcionite variant.
I've also made the case - with prominent support here at the forum - that Against Marcion goes back to a lost-ur text. The introduction to Against Marcion makes that explicit. While the author said he's restored the text to its original integrity after 'corruption' occurred when it ended up in the hands of an 'apostate' I doubt that happened. The text was altered from its original integrity by Irenaeus. The Luke-based arguments are not original to the text. Nevertheless certain anomalies remain - namely the Galatians-first Pauline canon which is known to have been preserved in the Near East.
The importance for us here is that since the Galatians-first canon is not Marcionite but representative of the tradition associated with Ephrem, not only is the original author's use of a gospel harmony confirmed but more importantly that he is using his Galatians not Marcion's Galatians. While I can't prove that Marcion's Galatians didn't have the bit about those coming from James slipping into Paul's church (i.e. infiltration from Jewish Christians) it is strongly inferred by the lack of mention of Marcion in the section in Justin's use of Origen's Commentary on Galatians which makes frequent allusion to Marcionite exegesis of Galatians.
Moreover, circumstantially, it is worth noting that Galatians 2 forms the basis of Against Marcion's thrust against Marcion. In Book 4 after making the case that Marcion falsified Luke (by no means a proven hypothesis) the author (who is necessarily not the original author according to my understanding) goes on to say the following:
It is another matter if in Marcion's opinion the Christian religion, with its sacred content, begins with the discipleship of Luke. However, as it was on its course even before that, it certainly possessed an authoritative structure by means of which it reached even to Luke: and so with the support of its evidence Luke also can find acceptance. But Marcion has got hold of Paul's epistle to the Galatians, in which he rebukes even the apostles themselves for not walking uprightly according to the truth of the gospel,a and accuses also certain false apostles of perverting the gospel of Christ: and on this ground Marcion strives hard to overthrow the credit of those gospels which are the apostles' own and are published under their names, or even the names of apostolic men, with the intention no doubt of conferring on his own gospel the repute which he takes away from those others.
To a naive observer, one who is unaware of the ACTUAL Marcionite understanding, this argument sounds quite convincing. Marcion read Galatians 2 and expanded on Paul's rather limited objection to 'Jewish Christianity' exaggerating his opposition because Marcion knew and opposed the gospels of Matthew, Mark and John.
Not only is this a stupid position for Marcion (Luke is clearly not an ur-gospel; the opening lines disprove that) it goes against what we know to be the actual position of the Marcionites from Adamantius. It is there we hear quite clearly that the Marcionites denied that any of the gospels were written by disciples. To that end, it is hard to believe Marcion would have read Galatians 2 and recognized that Matthew (for Matthew is that only gospel that makes sense in this context) was being promoted against Luke. After all, the Marcionites believed that Matthew was pseudepigraphal. To that end, we have to begin to suspect that exact opposition of Against Marcion occurred - Galatians 2 was interpolated into Paul's warning against the corruption of his gospel in order to contextualize and ultimately 'soften' the import of these words.
For the Marcionites clearly believed that Paul wrote a written gospel. This is without a doubt. As such the claim about Marcion stealing Luke is contrived. Moreover it subordinates the gospel of Luke because it was 'apostolic' (a contrived use of the original adjective) rather than one written by an apostle. Indeed if we look carefully at what follows in Book 4, rather than be as certain as he first claims, the author goes on to admit he has no idea what the Marcionites actually claimed:
And yet, even if there is censure of Peter and John and James, who were esteemed as pillars,b the reason is evident. It was that they appeared to be altering their manner of life through respect of persons. Yet since Paul himself made himself all things to all men so that he might gain them all,c Peter too may well have had this in mind in acting in some respect differently from his manner of teaching. And besides, if false apostles also had crept in, their character too is indicated: they were insisting on circumcision, and the Jewish calendar. So it was not for their preaching but for their forms of activity that they were marked down as wrong by Paul, though he would no less have marked them wrong if they had been in any error on the subject of God the Creator, or of his Christ. Therefore we have to distinguish between the two cases. If Marcion's complaint is that the apostles are held suspect of dissimulation or pretence, even to the debasing of the gospel, he is now accusing Christ, by thus accusing those whom Christ has chosen. If however the gospel which the apostles compared with Paul's was beyond reproach, and they were rebuked only for inconsistency of conduct, and yet false apostles have falsified the truth of their gospels, and from them our copies are derived, what can have become of that genuine apostles' document which has suffered from adulterators—that document which gave light to Paul, and from him to Luke? Or if it has been completely destroyed, so wiped out by a flood of falsifiers as though by some deluge, then not even Marcion has a true one. Or if that is to be the true one, if that is the apostles', which Marcion alone possesses, then how is it that that which is not of the apostles, but is ascribed to Luke, is in agreement with ours? Or if that which Marcion has in use is not at once to be attributed to Luke because it does agree with ours—though they allege ours is falsified in respect of its title—then it does belong to the apostles. And in that case ours too, which is in agreement with that other, no less belongs to the apostles, even if it too is falsified in its title (i.e. the 'apostolic').
What the author finds difficult to reconcile - i.e. the Marcionite habit of calling the gospel 'apostolic' (sing.) - is plainly comprehensible. The text was identified as 'apostolic' because it belonged to the apostle - i.e. Paul - viz. it was his written gospel.
As such if the author was able to lie about 'Luke' being involved in the transmission of the Marcionite gospel he is more than capable of lying about an interpolated section in Galatians 2. Note the number of times 'if' is used in the section 'even if' there is this, 'if' false apostles crept in, 'if' Marcion's complaint is this, 'if' the Marcionites claimed the 'apostolic gospel' was beyond reproach, 'if' the Marcionites claim this 'apostolic' gospel was destroyed, 'if' the Marcionites claimed a number of other things - why doesn't the author know what the Marcionites said about Galatians 2 if he attributes that their arguments about Paul's warning about the corruption of the gospel come from that section.
'If' is used ten times in this section. This belies the fact that the author in fact does not know that the Marcionites used this section to make their case. He has no idea how or 'if' the Marcionites used this section. He's just found a useful way of limiting their claims by making a section where Paul makes up with Peter and tolerates and even subordinates himself and his tradition to the apostolic one. This is all lies. This was a massive interpolation for a deliberate purpose -
viz. to de-legitimize the central Marcion claim that 'the apostle' wrote the original gospel called by them 'the apostolic.'