The reason I have come to this conclusion is spelled out in previous posts. I am not sure that any other scholar has explicit put forward an argument that Celsus used Justin, nevertheless the idea does come up from time to time in discussions of early anti-heretical treatises - i.e. that Celsus is work is placed alongside Justin's work. What I have done to turn things around however is to stop scholarship's slavish devotion to the idea that Irenaeus's treatise must necessarily represent the source for the Philosophumena, a text usually attributed to his student Hippolytus (although the manuscript attributes the text to Origen). There is certainly a great deal of material common to both. Yet I have found good reason to suppose that both texts go back to Justin's lost Syntagma and in fact the Philosophumena more closely resembles this source that Against Heresies.
This is not the place to begin the process of explaining what happened to Against Heresies to have altered its composition. One obvious explanation is to notice that Epiphanius's version of Irenaeus's text must have been different enough from ours to allow him to attribute the 'redemption rite' in Book One Chapter 21 to another Valentinian group rather than the Marcosians. There are many other examples which come to mind. Both Against Heresies and the Philosophumena place Irenaeus's account of the Valentinians and the Marcosians in different place in the narrative. Against Heresies starts with these two accounts back to back, whereas the Philosophumena places it after the account of Simon and Helen.
It is also worth noting that Tertullian preserves a 'stand alone' addition of the work against the Valentinians (appropriately entitled 'Against the Valentinians') which differs markedly from our existing material both in terms of style, order and content. It is also overlooked that when Epiphanius cites large amounts of Irenaeus's work against the Valentinians he explicitly states his source as "Irenaeus against the Valentinans" rather than Against Heresies (Pan. 31.32.9). Of course the common anti-Valentinian the material used by the Philosophumena and Against Heresies differ from one another and each against the surviving Latin text. It would seem Photius's comment about various original 'lectures' of Irenaeus, no less that Epiphanius's own statement about a collection of 'writings of Irenaeus' (Pan. 34.1.8) point to the existence of original treatises preserved in various forms of reworking.
Moreover it is also worth noting that the Philosophumena only mentions Irenaeus's writings in relation to these Valentinian heresies. A different account of the heretics Basilides, Marcion and many others were used by this author. This suggest again that rather than having the five books of Irenaeus Against the Heresies he had access to earlier editions of Against Marcus, Against the Valentinians and a 'second edition' which combined these treatises into one edition. The author of the Philosophumena was a student of Hippolytus, and thus a grand-student as it were of Irenaeus. The current edition had probably went through several rewrites and incorporation of new material.
One of the obvious aims of this third century text was to defend Irenaeus's reputation. The Philosophumena explicitly says that the Marcosians came across Irenaeus's account and complained about his portrait of their sect - for also the blessed presbyter Irenaeus, having approached the subject of a refutation in a more unconstrained spirit, has explained such washings and redemptions, stating more in the way of a rough digest what are their practices. (And it appears that some of the Marcosians) on meeting with (Irenaeus' work), deny that they have so received (the secret rite just alluded to), but they have learned that always they should deny. Wherefore our anxiety has been more accurately to investigate, and to discover minutely what are the (instructions) which they deliver in the case of the first bath, styling it by some such name; and in the case of the second, which they denominate Redemption. But not even has this secret of theirs escaped (our scrutiny). For these opinions, however, we consent to pardon Valentinus and his school." (Phil. 6.37)
This testimony is indeed quite fascinated for it gives us a sense of the author's motivation for reworking the material Irenaeus's work was not well received by the Marcosians. But what was their specific objection? It would seem that they were trying to distance themselves from some or all of the account of the secret redemption baptism. They wanted attribute the rite to the Valentinians, but the author was having no part in it. Clearly though Epiphanius ultimately fell into the very error the Philosophumena attributes to the Marcosians - namely attributing the 'redemption rite' to a Valentinian group. What exactly is the difficulty here?
It would seem that the earliest edition of Irenaeus's work against the Marcosians attributed the separate redemption baptism rites to individuals who were later identified as 'Valentinians.' Epiphanius specifically cites what he calls "the work written against Marcus himself and his successors by the most holy and blessed Irenaeus ... word for word" (Phil. 34.1.8). This reference leaves little doubt that in its original form there existed a stand alone work against the Marcosians attributed to Irenaeus. At some point this 'work written against Marcus' was incorporated into a second work written against the Valentinians. 'Against the Valentinians' assumed the first in order, this was followed by the work against Marcus with the identification of various 'followers of Marcus' who adapted this original understanding.
As noted above, these critics must have argued that these men were Valentinians rather than followers of their master Marcus. Hippolytus will have none of it arguing that "the blessed presbyter Irenaeus has powerfully and elaborately refuted the opinions of these ... the worthless opinions of these men have been sufficiently explained, and that it has been clearly proved whose disciples are Marcus and Colarbasus, who were successors of the school of Valentinus." (Phil 6.50) The Marcosians did not accept the idea that their Marcus was a Valentinian either clearly. Great efforts were made to reinforce this view including having the narrative begin with Valentinus and followed by Marcus. These two originally separate treatises of Irenaeus's were staggered back to back to give that impression but it was not historically accurate.
The Marcosians must have been understood to be a group in their own right. Epiphanius not only explicitly confirms Colorbasus and Heracleon to have been disciples of Marcus rather than Valentinus, it is well established as noted that Epiphanius takes the sacramental practices of the Marcosians described in AH 1.21.3-5 as referring to the followers of Heracleon. Clearly then the author of the Philosophumena also saw the same argument reflected in his copy of the original work Irenaeus wrote against the Marcosians and argued strenuously that Heracleon was a Valentinian not a Marcosian.
Indeed while we have gotten used to the idea of Valentinus being the head of all the other sects the idea it cannot be denied that in Tertullian's edition of Against the Valentinians, the whole section about the Marcosians is absent. Not only is a wholly separate work that was fused on to the end of Against the Valentinians, it was done so with the deliberate purpose of establishing Valentinus rather than Marcus as the founder of the gnostic tradition. Yet Epiphanius not only mention a separate 'work against Marcus and his successors,' (Pan. 34.1.8) but Colorbasus is identified specifically as his partner. (ibid 35.1.2) Colorbasus "drew on Marcus' sorcery ... [and] whose ideas were the same as his, their sect being like a two-headed snake. But later, like a head cut off a snake’s body and still breathing, he did fatal harm to many by showing them something supposedly greater and more authentic than his contemporaries and predecessors had."
Heracleon is only identified by Epiphanius as one step removed from Marcus, described as Colorbasus's successor and Marcus's spiritual heir:
Heracleon—and the Heracleonites who, as said, derive from him—like Marcus and certain of his predecessors makes allegations about the Ogdoads, I mean the upper and the lower. Then, too, he takes the same view of the syzygies of the thirty Aeons. He too alleges that the Father of all on high, whom he also called 'Depth,' is a man. He too wants to say that the Father is neither male nor female, but that the Mother of all, whom he calls both Silence and Truth, is derived from him. And derived from her is the second Mother, the one who had the lapse of memory, whom he too calls Achamoth. From her all things were brought into being defectively. But he too intends to say more than his predecessors, and it is this. He 'redeems' those of their people who are dying and have reached the actual point of death, taking his cue from Marcus, but no longer doing it in Marcus' way—for his part handling it differently by redeeming his dupes at the point of death, if you please. 'For sometimes some of them will mix oil with water, and apply it to the head of the dying; others apply the ointment known as balsam, and water.' But they have in common the invocation as Marcus before him composed it, with the addition of certain names. (ibid 36.2.1 - 5)
Scholars have never been able to explain where Epiphanius is getting his ideas about Colorbasus and Heracleon as disciples of Marcus. Their unconscious assumption is that Irenaeus's Against Heresies 'is the original text.' But if we look carefully there are strong reasons for assuming that it is fact a thoroughly edited version of the original.
Moreover we should note that buried in the existing manuscript of Against Heresies are utterly inexplicable references to Marcus and Colorbasus which must go back to an original work very close to what is preserved in Epiphanius's Panarion. Irenaeus says - like Epiphanius allusion to Colorbasus being Marcus's 'partner' - that "this Marcus then, says that he and he alone in his uniqueness has become the womb and receptacle of Colorbasus' Silence." (AH 1.14.1) Though the Philosophumena denies the pre-existent claims about Heracleon being a Marcosian, he also decides to omit the offending section about Heraclean redemption practices adapted from Marcus.
As we already noted the Marcosians were using this section to argue that it was Valentinians who followed secret rites not them. Perhaps this is why the Philosophumena goes to such extreme lengths to emphasize the idea that the Marcosians were indeed the only group associated with this practice:
And subsequent to the (first) baptism, to these they promise another, which they call Redemption. And by this (other baptism) they wickedly subvert those that remain with them in expectation of redemption, as if persons, after they had once been baptized, could again obtain remission. Now, it is by means of such knavery as this that they seem to retain their hearers. And when they consider that these have been tested, and are able to keep (secret the mysteries) committed unto them, they then admit them to this (baptism). They, however, do not rest satisfied with this alone, but promise (their votaries) some other (boon) for the purpose of confirming them in hope, in order that they may be inseparable (adherents of their sect). For they utter something in an inexpressible (tone of) voice, after having laid hands on him who is receiving the redemption. And they allege that they could not easily declare (to another) what is thus spoken unless one were highly tested, or one were at the hour of death, (when) the bishop comes and whispers into the (expiring one's) ear. And this knavish device (is undertaken) for the purpose of securing the constant attendance upon the bishop of (Marcus') disciples, as individuals eagerly panting to learn what that may be which is spoken at the last, by (the knowledge of) which the learner will be advanced to the rank of those admitted into the higher mysteries. And in regard of these I have maintained a silence for this reason, lest at any time one should suppose that I was guilty of disparaging these (heretics). For this does not come within the scope of our present work, only so far as it may contribute to prove from what source (the heretics) have derived the standing-point from which they have taken occasion to introduce the opinions advanced by them. (Phil. 6.36)
The highlighted section is not nearly as explicit in Irenaeus's Against Heresies. One can make a powerful argument that it must have been recognized then there was some ambiguity in Irenaeus's original work which the author has sought to address. The idea that Colorbasus and Heracleon were really Marcosians rather than Valentinians is only one of the things corrected by the author - the Marcosian employment of secret practices of baptism happen to be another.
It is worth noting that if we look at Irenaeus's account of the Marcosians there a number of references to Marcus having 'disciples,' individuals whose names have been removed from the existing text. The account begins with the notion that Marcus "is a perfect adept in magical impostures, and by this means drawing away a great number of men, and not a few women, he has induced them to join themselves to him, as to one who is possessed of the greatest knowledge and perfection, and who has received the highest power from the invisible and ineffable regions above." (AH 1.13.1) It is worth noting that these disciples must be the same who are identified in the above section from the Philosophumena "hoodwinking therefore multitudes, he led on (into enormities) many (dupes) of this description who had become his disciples, by teaching them that they were prone, no doubt, to sin, but beyond the reach of danger, from the fact of their belonging to the perfect power, and of their being participators in the inconceivable potency." (Phil. 6.36)
Irenaeus also speaks of "some of (Marcus's) disciples, too, addicting themselves to the same practices, have deceived many silly women, and defiled them. They proclaim themselves as being "perfect," so that no one can be compared to them with respect to the immensity of their knowledge, nor even were you to mention Paul or Peter, or any other of the apostles. They assert that they themselves know more than all others, and that they alone have imbibed the greatness of the knowledge of that power which is unspeakable. They also maintain that they have attained to a height above all power, and that therefore they are free in every respect to act as they please, having no one to fear in anything. For they affirm, that because of the 'Redemption' it has come to pass that they can neither be apprehended, nor even seen by the judge." (AH 1.13.6)
The point of course is that it is important to see that the report doesn't speak of 'Marcus's redemption' as much as it opens the door to a description of the theories of redemption associated with Marcus's many disciples. Indeed, in the passage just cited we hear Irenaeus over and over again speaks of the exact nature of what 'they proclaim' and 'they assert' and 'they maintain' and 'they affirm.' The parallel section in the Panarion reads "certain of his disciples too, who wander about in the same area (Asia), have deceived and seduced many women by proclaiming themselves so perfect that no one can equal the greatness of their knowledge—not even Paul or Peter or any other apostle. They claim that they know more than everyone, that they alone have drunk in the greatness of the knowledge of the ineffable power, and that they are higher than any power. Hence they can do everything freely and have no fear in anything. Because of their redemption they have become untouchable by the judge, and invisible to him. But even if he were to apprehend them they would stand before him with their redemption and say thus ..."
It would seem quite clear then that at one time Marcus - rather than Valentinus - was taken to be the earliest, greatest and most influential heretics.