Ehrman on Why We Shouldn't Believe Epiphanius as a Textual Critical Source

So let's recount the principle reasons why Epiphanius shouldn't be believed when he repeatedly claims in the Panarion that he has assembled all of the variants in the Bible of Marcion.  He begins by making it crystal clear that:

[s]ome years ago, to find what falsehood this Marcion had invented and what his silly teaching was, I took up his very books (αὐτὰς δὴ τὰς τοῦ προειρημένου βίβλους) which he had mutilated, his so-called Gospel and Apostolic Canon (τό τε παρ' αὐτῷ λεγόμενον εὐαγγέλιον καὶ τὸ ἀποστολικὸν καλούμενον). From these two books (δύο βιβλίων) I made a series of extracts and selections (παρ' αὐτῷ ἐξανθισάμενος καὶ ἀναλεξάμενος καθ' εἱρμὸν) of the material which would serve to refute him (τὰ ἐλέγξαι αὐτὸν δυνάμενα), and I wrote a sort of outline for a treatise arranging the points in order (ἐδάφιόν τι συντάξεως ἐποιησάμην  ἀκολούθως τάξας), and numbering each saying one, two, three (κεφάλαια καὶ ἐπιγράψας ἑκάστῃ ῥήσει ˉα ˉβ ˉγ). And in this way I went through all of the passages (καὶ οὕτως ἕως τέλους διεξῆλθον or perhaps 'I went through until the end ...) in which it is apparent that, foolishly, he still retains against himself these leftover sayings of the Saviour and the apostle. For some of them had been falsely entered by himself, in an altered form and unlike the authentic copy of the Gospel and the meaning of the apostolic canon. But others were exactly like both the Gospel and Apostle, unchanged by Marcion but capable of completely demolishing him. (Panarion 10.2 - 5)

This is quite explicit.  However the section of the Panarion dealing with the Borborites is similarly explicit but Ehrman concludes that Epiphanius is lying about his attentiveness.

Regarding the sect that is called 'the filthy' (hardly a name the sect gave themselves assuming they even existed) Ehrman notes that Epiphanius claims that "he had some contact with the group as a young man–was nearly seduced into it."  But Ehrman goes on to debunk the claims made by Epiphanius from what is inferred to be 'direct contact' with their religious practices:

it is sometimes claimed that he had special access to their liturgical practices. But this is scarcely plausible. Epiphanius indicates that he spurned the advances of the two attractive Phibionite women before being drawn into their orb. This must mean that he was never present for any of the ritual activities. And it defies belief that missionaries would inform outsiders about the scandalous and reprehensible activities of the group before they were admitted into the inner circle. Potential converts were not likely to be won over by accounts of ritualistic consumption of fetuses.

So Ehrman rightly concludes that Epiphanius is lying about his firsthand knowledge of the sect here and more importantly when it comes to a splinter group called the Phibionites and his claim to have seen - and later cites from - their 'books.'

Here is what Ehrman says on the subject:

Epiphanius, as we have seen, does claim to have read the Phibionites' literature, and this claim is sometimes taken to substantiate his account, even though he himself both provides the account and makes the claim. Here as both provides the account and makes the claim. Here as always Epiphanius must be taken with a pound of salt. The books of the Phibionites could not have been widely circulated outside the group—at least any books that documented their scandalous activities. So possibly Epiphanius read some of their theological or mythological treatises, and drew (or conjured up) his own conclusions. But did he read the Greater Questions of Mary and quote it accurately in his Panarion? There is evidence that some such book did at one time exist: it is at least mentioned elsewhere, although there is no evidence that any other author of a surviving work actually had seen it.32 But nowhere else, outside of Epiphanius, are we given any indication of its contents. The episode that Epiphanius cites of Jesus engaging in illicit sex, coitus interruptus, and consumption of his own semen coincides perfectly well with Epiphanius' description of the activities of the Phibionites themselves. Moreover, Epiphanius almost certainly fabricated the accounts of these activities: he had never seen them, no one from within the group would have told him about them, they could not have been described in their other literature, and they stand at odds with what we do know of the ethical impulses of all other Gnostic groups from antiquity. On these grounds I would propose that Epiphanius made up the account of the Greater Questions of Mary.

While I agree with most of what Ehrman says I happen to think that Epiphanius's relationship with this sexualized material is a little more complex.  I think that there are good grounds for believing that the second century Christian 'historian' Hegesippus is the source for this material.  Perhaps not the Questions of Mary but certainly the description of Christian orgies.  It followed immediately after the curtailed reference to Marcellina shared by Irenaeus.

Yet the point surely is that Epiphanius is lying about his first knowledge of sectarians in order to make his research sound more authoritative.  We see this thing happen all the time in humanities.  Castaneda's Teachings of Don Juan is one such example.  Castanada received a PhD for his work with Yaqui Indians nevertheless he later embellished this familiarity to create wholly fictitious narratives related to his travels in Mexico.  In the case of Epiphanius's claims to have the two books of Marcion in front of him while composing his 'outline' of textual variants, the reason why he established the lie is obvious.

The reality was that he gleaned textual variants from the accounts directed 'Against Marcion' of which Tertullian's was one of the more prominent sources.  These sources were likely not claiming to be 'firsthand sources' for the Marcionite Bible.  Tertullian never claims to have had Marcion's gospel or apostle.  As such developing a compendium from second or third-hand witnesses to the Marcionite Bible hardly inspires much confidence.  Assembling such a list and then claiming - as the 'icing on the cake' as it were - that you did no so from a 'firsthand source' i.e. the Marcionite Bible is awe-inspiring.

Yet there is more ...