It's Proven! Tertullian Says the Marcionites Appealed to the ΙΣ = איש Formula From the Pages of Genesis

This is the chapter which nails our hypothesis (it is important to note that the same material is reused in Tertullian's De Carne Christi chapter 3).  Tertullian confirms that the Marcionites 'borrowed from the Jews' with respect to their understanding of who ΙΣ is.  Already when the idea of Marcionite appropriation from Judaism comes up we know immediately we are on to something interesting.  The Marcionites effectively said that ΙΣ was man without human flesh and in order to prove their point they cited the example of the anashim = (plural form of איש) of Genesis 18 and other passages.  In effect then we get our foot into the door of the Marcionite ΙΣ = איש

Let the heretic now give up borrowing poison from the Jew,— the asp, as they say, from the viper: let him from now on belch forth the slime of his own particular devices, as he maintains that Christ was a phantasm: except that this opinion too will have had other inventors, those so to speak premature and abortive Marcionites whom the apostle John pronounced antichrists, who denied that Christ was come in the flesh, yet not with the intention of setting up the law of a second god—else for this too they would have been censured by the apostle —but because they had assumed it incredible that God should take to him human flesh.

So Marcion, even more of an antichrist, seized upon this assumption, being better equipped in fact for denial of Christ's corporal substance, in that he had postulated that even Christ's god was neither the creator of flesh nor would raise it to life again—in this too supremely good, and entirely divergent from the lies and deceptions of the Creator. And that is why his Christ, so as not to tell lies, or to deceive, and in this fashion perhaps be accounted as belonging to the Creator, was not that which he appeared to be, and told lies about what he was—being flesh and not flesh, man and not man, and in consequence a Christ who was god and not god.

For why should he not also have been clothed in a phantasm of god? Or can I believe what he says of his more recondite substance, when he has deceived me about that which was more evident? How shall he be accounted truthful about the secret thing, who has been found so deceptive about the obvious ? How can it have been that by confusing with- in himself truth of the spirit with deceit of the flesh, he conjoined that fellowship of light, which is truth, and deception, which is darkness, that the apostle says is impossible?

Also, now that it is found to be a lie that Christ was made flesh, it follows that all things that were done by means of Christ's flesh were done by a lie, his meetings with people, his touching of them, his partaking of food, his miracles besides. For if by touching some- one, or being touched by someone, he gave freedom from sickness, the act performed by the body cannot be credited as truly performed apart from the verity of the body itself. It was not feasible for anything solid to be performed by that which is void, anything full by that which is empty. Putative constitution, putative activity: imaginary operator, imaginary operations.

Thus also the sufferings of Marcion's Christ will fail to find credence: one who has not truly suffered, has not suffered at all, and a phantasm cannot have truly suffered. Consequently God's whole operation is overthrown. There is a denial of Christ's death, the whole weight and value of the Christian name, that death which the apostle so firmly insists on, because it is true, declaring it the chief foundation of the gospel, of our salvation, and of his own preaching. For I delivered unto you, he says, first of all, that Christ died for our sins, and that he was buried, and that he rose again the third day. But if his flesh is denied, how can his death be affirmed? For death is the particular experience of flesh, which by means of death is turned downwards into the earth from which it was taken: such is the law of its own Creator.

But if the death is denied, as it is when the flesh is denied, neither can there be assurance of the resurrection. By whatever reasoning he did not die, by the same reasoning he did not rise again: which was that he had not the substance of flesh, to which death appertains, and likewise resurrection. But further, if doubt is cast upon Christ's resurrection, ours also is overthrown: for if Christ's is not valid, neither can that be valid for the sake of which Christ came. For just as those who said there was no resurrection of the dead are con- futed by the apostle from the resurrection of Christ, so also, if Christ's resurrection fails, the resurrection of the dead is also taken away. And so also our faith is vain, and vain is the apostles' preaching.  They are also found false witnesses of God, because they have borne witness that he has raised up Christ, whom he has not raised up. And we are yet in our sins. And those who are fallen asleep in Christ, have perished—no doubt they will rise again, but in a phantasm perhaps, as Christ did.

If in this inquiry you think you can set against me the Creator's angels, alleging that they also, when in converse with Abraham and Lot, were in a phantasm, evidently of putative flesh, and yet really met with them, and partook of food, and performed the task committed to them, my answer be will first, that you have no claim upon the evidences of that God whom you are concerned to depose. For, the more superior and the more perfect the character of the god you are commending, the more unbecoming to him are evidences belonging to that other: for unless he is entirely diverse from him he cannot be in any sense better or more perfect.

Secondly, take note besides that we do not admit your claim that in those angels the flesh was putative: it was of veritable and complete human substance. For if it was not difficult for God to display true perceptions and activities in putative flesh, much easier did he find it to provide true perceptions and activities with true substance of flesh, the more so as he is himself its particular creator and maker.

Now your god, seeing that he has never produced any flesh at all, may quite reasonably perhaps have brought in a phantasm of something he had not the ability to make the truth of. But my God, who reshaped into the quality we know, that flesh which he had taken up out of clay— it was not yet conceived of conjugal seed, yet was already flesh— was no less able out of any material whatsoever to construct flesh for angels as well: he had even built up the world out of nothing into all these various bodies, and had done this with a Word.

And truly, if your god promises to men some time the true substance of angels—They will, he says, be as the angels—why should not my God too have granted to angels the true substance of men, from wheresoever he may have taken it? Since you for your part will not answer me when I ask from whence that angelic substance you speak of is to be taken, no more is required of me than to affirm as a fact, which is in keeping with God's dignity, the truth of that object which he presented to three witnesses, the senses of sight, and touch, and hearing. God finds it more difficult to tell lies than to bring into existence veritable flesh, from whatsoever source, even without the process of birth.

There are yet other heretics, who state that if in the angels that flesh had been truly human it would have needed to pass through human birth: to these we give in answer a firm reason why it was both truly human yet exempt from birth. It was truly human for the sake of the truth of God, who is a stranger to all lying and deceit, and because the angels could not have been received by men on human terms if they had not been in human substance: yet it had not passed through birth because Christ alone had the right to become incarnate of human flesh, so that he might reform our nativity by his own nativity, and thus also loose the bands of our death by his own death, by rising again in that flesh in which he was born with intent to be able to die.

For this reason he too on that occasion appeared along with the angels in Abraham's presence, in flesh veritable indeed though not yet born, because it was not yet to die, though it was even then learning to hold converse among men. Even more so the angels, who were never by God's intention to die for us, had no need to receive their brief experience of flesh by means of birth, because they were not intending to lay it down by means of death: yet from wheresoever it was they acquired it, and in whatsoever manner they finally disposed of it, they certainly did not tell lies about it. If the Creator maketh his angels spirits and his attendants a flaming fire, no less truly spirits than truly fire, he is the same who also made them truly flesh, so that we may now set it on record, and report back to the heretics, that the promise of some time reforming men into angels is made by that god who of old time formed angels into men. [Tertullian, Adv. Marc. 3.8, 9] 

Notice that when we take a look at the parallel section in De Carne Christi the reference to the איש in Genesis 32 has been deliberately removed from this discussion.  It reads (after much the same introduction):

You have read at one time, and believed it, that the Creator's angels were changed into human shape, and that the bodies they were clothed with were of such verity that Abraham washed their feet, and that by their hands Lot was snatched away from the men of Sodom, and an angel also having wrestled with a man with the whole weight of his body desired to be let go, so fast was he held.  Well then, that which was permitted to the angels of the inferior God when changed into human corporeity, the faculty of none the less remaining angels--will you deny this to the more mighty God, as though his Christ had not the power, when truly clothed with manhood, of continuing to be God? Or did perhaps those angels too become visible as a phantasm of flesh? No, this you will not dare to say. For if in your view the Creator's angels are as Christ is, Christ will belong to that God whose angels are such as Christ is. If you had not maliciously rejected some and corrupted others of the scriptures which oppose your views, the Gospel of John would in this matter have put you to rout when it proclaims that the Spirit in the body of a dove glided down and settled upon our Lord [De Carne Christi 3]

I'd say that our argument is on very firm foundation indeed.  Did not expect to find this supporting evidence for the formula ΙΣ = איש in Tertullian's anti-Marcionite material!