The original understanding promoted by Irenaeus was that Marcion was originally the disciple of Cerdo (= the 'fox'). This is preserved also in Book Four of Tertullian's Against Marcion:
Thus, Marcion brought nothing good out of Cerdon's evil treasure; nor Apelles out of Marcion's. [Tertullian Against Marcion 4.17]
There is no mention of Simon Magus in Book Four. In Hippolytus's wholly original account of Marcion 'stealing' his theology from Empedocles Simon is nowhere referenced until a quick gloss by a later editor to somehow tie the original material back to the edited edition of the Five Books Against Heresies:
But one Cerdon himself also, taking occasion in like manner from these (heretics) and Simon, affirms that the God preached by Moses and the prophets was not Father of Jesus Christ. For (he contends) that this (Father) had been known, whereas that the Father of Christ was unknown, and that the former was just, but the latter good. And Marcion corroborated the tenet of this (heretic) in the work which he attempted to write, and which he styled Antitheses. And he was in the habit, (in this book,) of uttering whatever slanders suggested themselves to his mind against the Creator of the universe. In a similar manner likewise (acted) Lucian, the disciple of this (heretic). [Philosophumena 7.25]
Nevertheless I hope it will obvious to any who have read the original material that the attempt here to bring Marcion under the umbrella of Simonian heretics was done at a later date. We know this also because Photius makes clear that the original Syntagma of Hippolytus traced the origins of the heresies back to Dositheus rather than Simon.
I think the same thing happened in the surviving editions of Justin's Apology (which are almost universally acknowledged to have survived in a corrupt form). What appears here with respect to Marcion is a later insertion as I will demonstrate here:
And, thirdly, because after Christ's ascension into heaven the devils put forward certain men who said that they themselves were gods; and they were not only not persecuted by you, but even deemed worthy of honours. There was a Samaritan, Simon, a native of the village called Gitto, who in the reign of Claudius Cæsar, and in your royal city of Rome, did mighty acts of magic, by virtue of the art of the devils operating in him. He was considered a god, and as a god was honoured by you with a statue, which statue was erected on the river Tiber, between the two bridges, and bore this inscription, in the language of Rome:— Simoni Deo Sancto, To Simon the holy God. And almost all the Samaritans, and a few even of other nations, worship him, and acknowledge him as the first god; and a woman, Helena, who went about with him at that time, and had formerly been a prostitute, they say is the first idea generated by him. And a man, Menander, also a Samaritan, of the town Capparetæa, a disciple of Simon, and inspired by devils, we know to have deceived many while he was in Antioch by his magical art. He persuaded those who adhered to him that they should never die, and even now there are some living who hold this opinion of his. And there is Marcion, a man of Pontus, who is even at this day alive, and teaching his disciples to believe in some other god greater than the Creator. And he, by the aid of the devils, has caused many of every nation to speak blasphemies, and to deny that God is the maker of this universe, and to assert that some other being, greater than He, has done greater works. All who take their opinions from these men, are, as we before said, called Christians; just as also those who do not agree with the philosophers in their doctrines, have yet in common with them the name of philosophers given to them. And whether they perpetrate those fabulous and shameful deeds — the upsetting of the lamp, and promiscuous intercourse, and eating human flesh— we know not; but we do know that they are neither persecuted nor put to death by you, at least on account of their opinions. But I have a treatise against all the heresies that have existed already composed, which, if you wish to read it, I will give you. [1 Apology 26]
This section, in my opinion originally read as follows in the earlier lost manuscripts:
And, thirdly, because after Christ's ascension into heaven the devils put forward certain men who said that they themselves were gods; and they were not only not persecuted by you, but even deemed worthy of honours. There was a Samaritan, Simon, a native of the village called Gitto, who in the reign of Claudius Cæsar, and in your royal city of Rome, did mighty acts of magic, by virtue of the art of the devils operating in him. He was considered a god, and as a god was honoured by you with a statue, which statue was erected on the river Tiber, between the two bridges, and bore this inscription, in the language of Rome:— Simoni Deo Sancto, To Simon the holy God. And almost all the Samaritans, and a few even of other nations, worship him, and acknowledge him as the first god; and a woman, Helena, who went about with him at that time, and had formerly been a prostitute, they say is the first idea generated by him. And a man, Menander, also a Samaritan, of the town Capparetæa, a disciple of Simon, and inspired by devils, we know to have deceived many while he was in Antioch by his magical art. All who take their opinions from these men, are, as we before said, called Christians; just as also those who do not agree with the philosophers in their doctrines, have yet in common with them the name of philosophers given to them. And whether they perpetrate those fabulous and shameful deeds — the upsetting of the lamp, and promiscuous intercourse, and eating human flesh— we know not; but we do know that they are neither persecuted nor put to death by you, at least on account of their opinions. But I have a treatise against all the heresies that have existed already composed, which, if you wish to read it, I will give you. [1 Apology 26]
The reference 'until this very day' is very telling because I think it will help us identify the third century Christian who served as the model for Marcion for the editor.
The same can be said of the second reference to Marcion in the Apology:
But the evil spirits were not satisfied with saying, before Christ's appearance, that those who were said to be sons of Jupiter were born of him; but after He had appeared, and been born among men, and when they learned how He had been foretold by the prophets, and knew that He should be believed on and looked for by every nation, they again, as was said above, put forward other men, the Samaritans Simon and Menander, who did many mighty works by magic, and deceived many, and still keep them deceived. For even among yourselves, as we said before, Simon was in the royal city Rome in the reign of Claudius Cæsar, and so greatly astonished the sacred senate and people of the Romans, that he was considered a god, and honoured, like the others whom you honour as gods, with a statue. Wherefore we pray that the sacred senate and your people may, along with yourselves, be arbiters of this our memorial, in order that if any one be entangled by that man's doctrines, he may learn the truth, and so be able to escape error; and as for the statue, if you please, destroy it.
Nor can the devils persuade men that there will be no conflagration for the punishment of the wicked; as they were unable to effect that Christ should be hidden after He came. But this only can they effect, that they who live irrationally, and were brought up licentiously in wicked customs, and are prejudiced in their own opinions, should kill and hate us; whom we not only do not hate, but, as is proved, pity and endeavour to lead to repentance. For we do not fear death, since it is acknowledged we must surely die; and there is nothing new, but all things continue the same in this administration of things; and if satiety overtakes those who enjoy even one year of these things, they ought to give heed to our doctrines, that they may live eternally free both from suffering and from want. But if they believe that there is nothing after death, but declare that those who die pass into insensibility, then they become our benefactors when they set us free from sufferings and necessities of this life, and prove themselves to be wicked, and inhuman, and bigoted. For they kill us with no intention of delivering us, but cut us off that we may be deprived of life and pleasure.
And, as we said before, the devils put forward Marcion of Pontus, who is even now teaching men to deny that God is the maker of all things in heaven and on earth, and that the Christ predicted by the prophets is His Son, and preaches another god besides the Creator of all, and likewise another son. And this man many have believed, as if he alone knew the truth, and laugh at us, though they have no proof of what they say, but are carried away irrationally as lambs by a wolf, and become the prey of atheistical doctrines, and of devils. For they who are called devils attempt nothing else than to seduce men from God who made them, and from Christ His first-begotten; and those who are unable to raise themselves above the earth they have riveted, and do now rivet, to things earthly, and to the works of their own hands; but those who devote themselves to the contemplation of things divine, they secretly beat back; and if they have not a wise sober-mindedness, and a pure and passionless life, they drive them into godlessness. [ibid 56 - 58]
I would again propose that this originally read:
But the evil spirits were not satisfied with saying, before Christ's appearance, that those who were said to be sons of Jupiter were born of him; but after He had appeared, and been born among men, and when they learned how He had been foretold by the prophets, and knew that He should be believed on and looked for by every nation, they again, as was said above, put forward other men, the Samaritans Simon and Menander, who did many mighty works by magic, and deceived many, and still keep them deceived. For even among yourselves, as we said before, Simon was in the royal city Rome in the reign of Claudius Cæsar, and so greatly astonished the sacred senate and people of the Romans, that he was considered a god, and honoured, like the others whom you honour as gods, with a statue. Wherefore we pray that the sacred senate and your people may, along with yourselves, be arbiters of this our memorial, in order that if any one be entangled by that man's doctrines, he may learn the truth, and so be able to escape error; and as for the statue, if you please, destroy it.
Nor can the devils persuade men that there will be no conflagration for the punishment of the wicked; as they were unable to effect that Christ should be hidden after He came. But this only can they effect, that they who live irrationally, and were brought up licentiously in wicked customs, and are prejudiced in their own opinions, should kill and hate us; whom we not only do not hate, but, as is proved, pity and endeavour to lead to repentance. For we do not fear death, since it is acknowledged we must surely die; and there is nothing new, but all things continue the same in this administration of things; and if satiety overtakes those who enjoy even one year of these things, they ought to give heed to our doctrines, that they may live eternally free both from suffering and from want. But if they believe that there is nothing after death, but declare that those who die pass into insensibility, then they become our benefactors when they set us free from sufferings and necessities of this life, and prove themselves to be wicked, and inhuman, and bigoted. For they kill us with no intention of delivering us, but cut us off that we may be deprived of life and pleasure.
For they who are called devils attempt nothing else than to seduce men from God who made them, and from Christ His first-begotten; and those who are unable to raise themselves above the earth they have riveted, and do now rivet, to things earthly, and to the works of their own hands; but those who devote themselves to the contemplation of things divine, they secretly beat back; and if they have not a wise sober-mindedness, and a pure and passionless life, they drive them into godlessness.
If my assumptions are accepted we can see why Irenaeus (or his later editor) has to go absolutely overkill to prove that Justin actually witnessed the existence of a man named 'Marcion' (because the Dialogue and other works make no reference whatsoever to him or his followers).
The Fourth Book of Against Heresies claims that Justin made a wild and crazy claim about his life long opposition to 'Marcion' saying:
In his book against Marcion, Justin does well say: "I would not have believed the Lord Himself, if He had announced any other than He who is our framer, maker, and nourisher. But because the only-begotten Son came to us from the one God, who both made this world and formed us, and contains and administers all things, summing up His own handiwork in Himself, my faith towards Him is steadfast, and my love to the Father immoveable, God bestowing both upon us."
It is worth noting that Justin cites this claim by Irenaeus that such a work by Justin existed twice in his Church History but as Schaff notes it was a work that Eusebius only knows through Irenaeus:
This work against Marcion is also mentioned by Irenæus, who quotes from it in his Adv. Hær. IV. 16. 2 (see below, chap. 18), and by Photius, Cod. 125. The work is lost, and we have only the single brief fragment preserved by Irenæus. It is possible that it formed a part of the larger Syntagma contra omnes Hæreses, mentioned by Justin in his Apol. I. 26 (see below), and it has been urged in support of this possibility that Irenæus nowhere mentions a work of Justin’s Against all Heresies, although it is highly probable that he made use of such a work (see Lipsius’ Quellen der ältesten Ketzergesch. and Harnack’s Zur Quellenkritik des Gnosticismus). It would seem that Irenæus is referring to this work when he mentions the Syntagma contra Marcionem. On the other hand, Photius mentions the work against Marcion and the one against all heresies as two separate works. He does not seem, however, to have had a personal knowledge of them, and is possibly only repeating Eusebius (Harnack says he is certainly doing so, Ueberlieferung d. griech. Apol. p. 150; but in view of the fact that he omits two works mentioned by Eusebius, this seems to be somewhat doubtful); and if this is so, no reliance is to be placed upon his report, for it is evident that Eusebius himself knew neither of the two works, and hence the fact that he distinguishes them has no significance. Although, therefore, it cannot be determined whether Justin wrote two separate works against heretics, it is quite probable that he did not.
The conduct of Eusebius in this connection is very peculiar. After mentioning the work against Marcion, he at once gives a quotation in such a way as to convey the impression that the quotation is taken from this work, but it is really taken from the first Apology. This makes it very probable that he had not seen this work against Marcion, a conclusion which is confirmed by its omission from the list of Justin’s writings given in chap. 18. It is claimed by many that Eusebius practices a little deception here, wishing to convey the impression that he knew a book which he did not know. This is not in accord with his usual conduct (as he seldom hesitates to confess his ignorance of any matter), and his general character for candor and honesty must be taken into account in deciding the case. He does not state directly that the quotation is taken from the work against Marcion, and it is possible that the seeming reference of it to that source was an oversight on his part. But it must be acknowledged, if that be the case, that he was very careless in making the quotation.
I think this is a highly significant discovery which points to the fact that there is something clearly amiss here. It isn't just Eusebius's knowledge of Justin which is at stake. The question of whether Justin really wrote a book against Marcion or indeed that he even know that a figure such as 'Marcion of Pontus' ever existed is really questionable. I would assume by contrast that Marcion was inserted into the testimony of the second century Fathers, developing as it did from an attempt to 'rehabilitate' the original heretical claims about the Alexandrian tradition of Mark.