My own personal suspicion is that many or most of the descriptions of heresies in Irenaeus were exaggerations. Rather than acknowledging that reality, Epiphanius went in the other direction when penning his anti-heretical Panarion. He went bigger and bolder, often 'taking things to the next level' in terms of heightening the scandal of the heresies and the authority of the original information 'condemning' them. As David Frankfurter succinctly puts it "Epiphanius inevitably exaggerates the extent of 'immoralities' occurring around him."
Yet there is more to his repurposing of the common source behind the Panarion and Tertullian's Against Marcion Books Four and Five than this. Epiphanius like many readers must have been puzzled by the Galatians first ordering of the original treatise. Epiphanius acknowledges immediately after his compilation of a number of textual variations from the Marcionite gospel that:
in further opposition to this heresiarch I also attach, to this arrangement (of texts) which has been laboriously accumulated against him by myself, such other texts as I find in his works, as in an arbitrary version of the apostle Paul's epistles; not all of them but some of them—(I have listed their names in the order of his Apostolic Canon at the end of the complete work)—and these mutilated as usual by his rascality.
In other words, he goes on to list 'some' but not all of the textual variants between the Marcionite canon and the Catholic text in a Romans first ordering without any detailed discussion of the disagreements.
In a later section of the same account against the Marcionites in his Panarion we see virtually the same list (though not exactly) of the textual variants in a Galatians-first ordering with the following introduction:
This is the publication of the treatise against Marcion based on the remains of the Gospel he preserves, which I have composed on his account and which, in my opinion, is adequate to expose his deceit. But I shall also go on to the next part, the texts from the Apostle which he still preserves, and which I have, again, selected in the same way. I have put the ones from the Epistle to the Galatians first, and keep that order throughout, for in Marcion's canon Galatians stands first. At the time I did not make my selection in his order but in the order of the Apostolic Canon, and put Romans first. But here I cite in accordance with Marcion's canon.
Why is the account developed in two separate treatments with two distinct orderings of the letters of Paul? Epiphanius claims that he is responsible for both these accounts, but we should find this hard to believe. It seems more likely that Epiphanius has taken over the work of someone else.
It is interesting that Epiphanus explicitly acknowledges in the introduction to the Romans-first section that what follows is not a complete list of variants. This is an unusually humble acknowledgement on the part of the Church Father who usually claims to know everything about a given subject. Surely if Epiphanius actually had the Marcionite canon in front of him, he could have ordered one of his underlings to simply tabulate all the significant variants. For instance Ephrem's mention of the change of location for the opening scene of the gospel from Nazareth to Bethsaida is ignored. How was this possible? It was allegedly right at the start of the gospel. The answer is that it was also ignored by Tertullian's source.
Similarly the omission of the Father raising Jesus at the beginning of Galatians mentioned by Origen (= Jerome) is also passed over in silence. Why so? Because Tertullian's source doesn't mention that one either. How could a variant in the first line of the alleged first epistle of the Marcionite canon have gone unnoticed by Epiphanius if he had the Apostolikon in front of him while tabulating textual variants between the heretical canon and 'our canon'? As such, it is impossible to believe that Epiphanius actually had a Marcionite canon in front of him.
In other words when Epiphanius says that these textual variants have "been laboriously accumulated against him [Marcion] by myself" what he is really saying is that he went through the original source behind Tertullian's Against Marcion Books Four and Five and perhaps a few other sources and compiled his textual variants accordingly. This is why the gospel list in both cases begin at the same point in the narrative - i.e. 'Go, show thyself unto the priest, and offer for thy cleansing, according as Moses commanded—that this may be a testimony unto you' instead of the Saviour's 'for a testimony unto them.' An actual transcription effort based on the Marcionite gospel would have placed the Nazareth to Bethsaida 'switch' before this.
Similarly the first variant in the Apostolikon in the Panarion is "Learn that the just shall live by faith. For as many as are under the Law are under a curse; but, The man that doeth them shall live by them." It cannot be mere coincidence that the first variant in Tertullian's treatment of the Apostolikon is the very same textual variant:
This then is my contention, that the faith belongs to that God to whom belongs the original pattern of the grace of faith. And again when he adds, For ye are all the sons of faith, it becomes evident how much before this the heretic's diligence has erased, the reference, I mean, to Abraham, in which the apostle affirms that we are by faith the sons of Abraham, and in accordance with that reference he here also has marked us off as sons of faith. Yet how sons of faith? and of whose faith if not Abraham's? For if Abraham believed God, and it was reckoned for righteousness, and thenceforth he had the right to be called the father of many gentile nations: and if we by believing God are the more thereby justified, as Abraham was, and the more obtain life, as the just man lives by faith: so it comes about that up above he pronounced us sons of Abraham, as the father of faith, and here sons of faith, that by which Abraham had received the promise of being the father of the gentiles. In this very fact of dissociating faith from circumcision, was not his purpose to constitute us sons of Abraham, of him who had believed while his body was still unmutilated? So then the faith of one god cannot obtain admittance to the rule laid down by another God, so as to credit believers with righteousness, cause the just to have life, and call the gentiles sons of faith. The whole of this belongs to that God in whose revelation it has already for a long time been known. [Adv Marc. 5.3]
Note the ambiguous reference here. Tertullian makes reference to the same line 'learn that the just shall live by faith' and then makes reference to Genesis. Earlier in the Panarion, Epiphanius does the very same thing:
Elenchus 1. The saying, 'Learn that the just shall live by faith,' as the apostle gives it is reference to an ancient scripture. Such things have been taken over by the apostle for our salvation, as statements from the Law and prophets (which are) about a new covenant and are conjoined with our hope.
The uncanny similarity in allusions - both extremely vague - alongside the mutual omission of previous and more significant textual variants show once and for all that both Tertullian and Epiphanius go back to a common source.
In plain language - Epiphanius is once again lying about having a source to 'strengthen' his inferences from an extremely problematic source - i.e. Irenaeus's lost Against Marcion.
