Well after criticizing Marcion the author of Against Marcion has a novel theory - it wasn't done for the disciples at all but to fulfill the promise of God to Moses to show himself to him:
I strongly suspect that the author is describing Marcion's understanding of the passage - i.e. that Marcion believed that Moses had been 'purchased' or carried off to heaven. This might explain the use of exodos in Luke 9:31. Moreover Ephrem seems to know a similar understanding when he writes:The Father gave to the Son new disciples,880 after that Moses and Elias had been exhibited along with Him in the honour of His glory, and had then been dismissed as having fully discharged their duty and office, for the express purpose of affirming for Marcion's information the fact that Moses and Elias had a share in even the glory of Christ. But we have the entire structure881 of this same vision in Habakkuk also, where the Spirit in the person of some882 of the apostles says, "O Lord, I have heard Thy speech, and was afraid." What speech was this, other than the words of the voice from heaven, This is my beloved Son, hear ye, Him? "I considered thy works, and was astonished." When could this have better happened than when Peter, on seeing His glory, knew not what he was saying? "In the midst of the two Thou shalt be known"----even Moses and Elias.883 These likewise did Zechariah see under the figure of the two olive trees and olive branches.884 [13] For these are they of whom he says, "They are the two anointed ones, that stand by the Lord of the whole earth." And again Habakkuk says, "His glory covered the heavens" (that is, with that cloud), "and His splendour shall be like the light----even the light, wherewith His very raiment glistened." And if we would make mention of885 the promise to Moses, we shall find it accomplished here. [14] For when Moses desired to see the Lord, saying, "If therefore I have found grace in Thy sight, manifest Thyself to me, that I may see Thee distinctly,"886 the sight which he desired to have was of that condition which he was to assume as man, and which as a prophet he knew was to occur. Respecting the face of God, however, he had already heard, "No man shall see me, and live." "This thing," said He, "which thou hast spoken, will I do unto thee." [15] Then Moses said, "Show me Thy glory." And the Lord, with like reference to the future, replied, "I will pass before thee in my glory," etc. Then at the last He says, "And then thou shall see my back."887 Not loins, or calves of the legs, did he want to behold, but the glory which was to be revealed in the latter days.888 He had promised that He would make Himself thus face to face visible to him, when He said to Aaron, "If there shall be a prophet among you, I will make myself known to him by vision, and by vision will I speak with him; but not so is my manner to Moses; with him will I speak mouth to mouth, even apparently" (that is to say, in the form of man which He was to assume), "and not in dark speeches."889 [16] Now, although Marcion has denied890 that he is here represented as speaking with the Lord, but only as standing, yet, inasmuch as he stood "mouth to mouth," he must also have stood "face to face" with him, to use his words,891 not far from him, in His very glory----not to say,892 in His presence. And with this glory he went away enlightened from Christ, just as he used to do from the Creator; as then to dazzle the eyes of the children of Israel, so now to smite those of the blinded Marcion, who has failed to see how this argument also makes against him.
Both reports are unfortunately vague (we should however be getting Nils Pederson's new translation of Ephrem any time now). There seems to have been a completely different gospel text. I don't think it was the same as our text.If again they returned and said to him "[As for] mankind, because thou art about to buy them, if thou didst take them beforehand, nothing hinders (?) : this mountain that thou hast gone up—and why ?—was this mountain also really required for thee ? And if it is required for thee, give |lxi the price of it, seeing thou hast gone up ; and if it is not intended by thee to buy the mountain, get down off it; why wilt thou stir up enmity for thyself with the Maker about nothing ? But the price of mankind will not be found by thee to give to the Maker, for He has given no pledge." If such words were [P. 90.] put forward (lit. were in the midst), and things similar to them, [then] it was for war that they had come to him. But if Isu came to (wage) war, he was not a good Being, for he did not purchase ... it would not be right for a good Being to injure, [l. 14.] much less those whom he had not yet even purchased ! And were it not that our Maker is good and there is no end to his kindness, He would surely, not have trusted the Stranger so as to give him men to accompany him, when as yet he had not paid their price to Him. Or was there, forsooth, a bargain ? And did Isu say to the Maker, 'Give me men, and I will not depart from Thy house, that is, Thy creation, until I pay Thee their price ' ? And did not the Maker learn from the descent of Isu that he was also to ascend, so that as there was no one who perceived him when he came down, in like manner he would remove those whom he wished to purchase and carry (them) off without any one perceiving him ? But perhaps the Maker [p. 91.] said to him these very things, and Isu returned answer to Him and said to Him, 'If I carry (them) off, as Thou thinkest, in virtue of that which I did when coming down, those souls which I am purchasing from Thee, how can I take them up without Thy consent ?'
And that we may not explore too far into the perverse tale of Marcion, this pact that Moses, etc., agreed on with the Stranger in the mountain,—the glory moreover, which He shewed them in the mountain, for what purpose (was it shewn) ? Can it be |lxii (that it was done) in order that He might shew them that what He gave was greater than what He received ? Then also Moses, etc., sold themselves to Him there, on account of that surpassing glory which they saw. And perhaps Isu too shewed them that glory on the mountain in order to incite Moses, etc., so that because Moses and Elijah were accustomed to that surpassing vision of the Maker Isu shewed them that (his glory) surpassed that of the Maker, in order that they might desire it eagerly on [P. 92.] account of its surpassing character. Well, then, in short, they made a bargain with him, because they had loved him.
And if thou sayest that neither for a sale nor for a bargain had Moses, etc., come to Him, then why had they come to Him ? Can it be that they had come to fight ? And very likely It is that men would come to fight against God ! And which of them is it who strikes (the blow), or which is it who is struck ? Or did he on this account take his Apostles with him and cause them to ascend (the mountain), in order that they might wage war with the Prophets ? And which of the sides conquered there or lost ? But that battle, what was it for ? Can it have been on account of the love of their Gods ? And why would not those Gods themselves contend for the love of mankind ? For if the Gods are at peace, why do they contend about mankind ? . . . [l. 39.] For if created things are from One, unadvisedly did Isu [1.42.] interpose, ... If they say that in truth the Stranger went up to heaven, see how much the Maker despised him and . . . [P.93.] against his disciples and against him [who said], 'This is my Son and my Beloved,' 33 [for] He had sent only two against them.
[1.10.] But [if] they say, ' If . . . is it not clear that because he was very strong on that account he did not overcome [him ? How] could two men [overcome] three ? [Were they just] two men — and not [both] alive, but one alive and one dead — to fight [a God] ! Was the Maker then really afraid to come, and on that account indeed did not come ? So that if He had come, He would have been killed ! Or can then a Divine Nature suffer pain, either |lxiii the Maker's or the Stranger's ? And if they did not suffer, why did the Maker not come against him ? Or can it be that He really knew that Moses, etc., would be sufficient to meet the attack of the Stranger, and therefore He did not come ? For lo, even the Stranger did not contend with them, and it is clear that he really perceived that they were stronger than he, and on that account he remained quiet (and refrained) from engaging in battle. And as to his preparing battle with the Maker, if [his desires hankered] 34 after men, why was he [lo,] unable to [P. 94.] create this ? And if to create men he was too weak, how much more was he too weak to wage war against God ! Again, the Stranger who proclaimed there, 'This is my Son and my Beloved,' whom did He wish to cause to hear (it) ? Can it be that He was calling to Moses, etc., that He might make them His disciples ? Or that He might warn them not to say anything to him (i.e. to Isu) ? And from which heavens did He call ? Was it from the heaven of the Maker ? And why did He descend to it ? If, as it were, on account of the aforesaid Maker the Stranger descended to it, then He did not snatch away men only but also the heaven. Or can it be that the Stranger purchased the angels who were in the heaven together with the heaven ? But if those who were above were not purchased by Him, why did He pass through their abodes ? But if (the voice) came that it might be a witness to the Son, who had no witness on earth, lo ! seeing that the voice came from the heaven of the Maker, who is to tell us that he is [P. 95.] not the Son of the Maker, in a case where the voice which came was coming from the heaven of the Maker, especially when the mountain was the mountain of the Maker, and the cloud of Moses, etc., belonged to the Maker, and the prophets likewise who were on the mountain (were the prophets) of the Maker ?
For if the voice had come from the heaven of the Stranger perhaps it would have been reasonable for us to think that in order that mankind might not be mistaken, owing to the mountain and the cloud and Moses, etc., on that account the voice was coming to them from the heaven of the Stranger, so as to overthrow the opinion which they had concerning Isu. But if even the voice |lxiv which came was from the heaven of the Maker, it did not by any means disown him (by asserting) that he was not the Son of the Maker, but it actually confirmed it that he is the Son of the Maker, and the servants of his Father's house, who had come to do him honour, were witnesses (thereto).
For if there had been a battle, the Maker would not have remained silent, He who even when there was another God did not [P. 96.] refrain from (saying) 'I am God and there is none beside.' And if when there were idols, whose nature showed (lit. answered) that they were not gods, He was proclaiming 'I am He and there is none beside,' (can we suppose that) in a case when a God was warring against a God the Creator went into a hiding-place, that the creation might go astray after the Stranger ? For if in connection with idols He had been silent, (yet) here it would be right for Him to cry out. How much more when He was not silent even towards dead idols ! But seeing that the questions relating to a war have, as in a (real) war, overcome and silenced the question of purchase, now that the tale about a war has come to an end, let us turn to the question of purchase. Explain to us then, What is the purchase which the Stranger made, and from whom did He purchase it ? And, moreover, by means of what did He purchase it ? And that thing by means of which he purchased that which He purchased, of what nature was it ? Was that which He gave of the nature of the aforesaid Good Being, or did He really create (something) and give (it) ? And was not that which the Stranger created fairer than that which the Maker created for Himself ? And if that which He (i.e. the Stranger) created for Himself was fairer, why did He [P. 97] sell unadvisedly and become a laughing-stock ? And if that which He gave was something smaller, the weakness of the Stranger was seen in His creative action. And how was the wise Just Being persuaded to give to the Stranger something great in return [P.98. 17] for something small ? . . . Was it . . . bodies that are from HULE that he bought, or souls ? And if it was souls, |lxv then why [did he not buy] the bodies ? . . . they say that [l. 32.] because the souls had been polluted (lit. had become turbid) He came to purify them. But if those souls were not polluted, then did not the Stranger who purchased them make a. mistake about them ? And even if the souls were polluted, on which account he came to buy them, [was he not alien to their nature ?]