I also believe that much of the work understanding the place of the 'Diatessaron' was done before the discovery of the Mar Saba document in 1958. I am confident that the document is indeed likely to be authentic. I also believe that its description of two gospels - one 'public' and literal the other 'secret' and mystical - should necessarily change our understanding of what the gospel called 'διὰ τεσσάϱων' which was traditionally associated with Tatian (c. late second century CE).
As I have noted here before, not only are there ancient references to a similar gospel by another name - Victor of Capua's διὰ πέντε - but we hear specific and numerous references to an 'Alexandrian Diatessaron' which apparently was to be distinguished from Tatian's text. I have to admit I was never that well attuned to the intricacies of Pythagorean mysticism. Nevertheless I can't help but notice that these allusions are particularly strong among Alexandrian writers starting with Philo (called 'the Pythagorean' by Clement), Clement and even Origen.
It is impossible not to see that Justin and Tatian on the one hand and Philo, Clement, Origen and the rest of the so-called 'Alexandrian tradition' on the other, could only have used the term διὰ τεσσάϱων if they had in mind another text - a διὰ πέντε - which necessarily represented a 'more spiritual' gospel (cf. to Theodore I.21 - 22). There can be no doubt that διὰ τεσσάϱων was used in common parlance at the time to mean 'less spiritual' especially as Clement clearly identifies this original less spiritual public gospel was written at Rome (where the Latin spiritus can be applied to alcoholic drinks).
That something called διὰ τεσσάϱων was necessarily 'less spiritual' or 'more sober' than compared to something else called διὰ πέντε is clear from Plutarch's the Morals Book Three where we read a preservation of the use of both terms in the popular language of the day:
When I had said this, Aristo cried out aloud, as his manner was, and said: I see well now that there is opened a return again of measures unto feasts and banquets; which measures, although they are most just and democratical, have for a long time (I wot not by what sober reason) been banished from thence, as by a tyrant. For, as they who profess a canonical harmony in sounding of the harp do hold and say, that the sesquialteral proportion produceth the symphony diapente (διὰ πέντε), the double proportion the diapason (διὰ πασῶν), and that the accord called diatessaron (διὰ τεσσάϱων), which is of all most obscure and dull, consisteth in the epitrite proportion (i.e. 4/3); even so they that make profession of skill in the harmonies of Bacchus have observed, that three symphonies or accords there are between wine and water, namely, diapente, diatrion (διὰ τϱιῶν), and diatessaron; and so they say and sing, — Drink either five or three, but not four. For the fifth has the sesquialteral proportion, three cups of water being mingled with two of wine; the third has the double proportion, two cups of water being put to one of wine; but the fourth answereth to the epitrite proportion of three parts of water poured into one of wine. Now this last proportion may be fit for some grave magistrates sitting in the council-hall, or for logicians who pull up their brows when they are busy in watching the unfolding of their arguments; for surely it is a mixture sober and weak enough. As for the other twain; that medley which carrieth the proportion of two for one bringeth in that turbulent tone of those who are half-drunk, "Which stirs the heart-strings never moved before" for it suffereth a man neither to be fully sober, nor yet to drench himself so deep in wine as to be altogether witless and past his sense; but the other, standing upon the proportion of three to two, is of all the most musical accord, causing a man to sleep peaceably and forget all cares, and, like the corn-field which Hesiod speaks of, which doth from man all curses drive, and children cause to rest and thrive, stilling and appeasing all proud and disordered passions within the heart, and inducing instead of them a peaceable calm and tranquillity.
These speeches of Aristo no one there would contradict, for it was well known that he spoke in jest. But I willed him to take a cup, and, as if it were a harp, to set and tune it to that accord and harmony which he so highly praised. Then came a boy close unto him, and offered him strong wine; but he refused it, saying with laughter, that his music consisted in theory, and not in practice of the instrument. Then my father added to what had been said, that the ancient poets gave two nurses to Jupiter, namely, Ite and Adrastea; one to Juno, Euboea; two, moreover, to Apollo, Alethea and Corythalea; while they gave many more to Bacchus. For, as it seemed to him, Bacchus was nursed and suckled by many Nymphs, because he had need of many measures of water (νύμφαι), to make him more tame, gentle, witty, and wise. [Plutarch, the Morals, III.9]
In short, I believe that it is highly probable that the obvious mystical interest in water vs. wine in the Christian sacraments also has something to do with this. We see a similar discussion emerge in Irenaeus's report on the heretic 'Mark' (cf. AH 1.14 - 15). My guess is that the distinction between διὰ τεσσάϱων and διὰ πέντε developed in a community which had two gospels as outlined in to Theodore. We should notice also that Celsus's discussion of the development of more than one gospel (Contra Celsum 2.27) - i.e. that "certain of the Christian believers, like persons who in a fit of drunkenness lay violent hands upon themselves, have corrupted the Gospel from its original integrity, to a threefold, and fourfold, and many-fold degree, and have remodelled it, so that they might be able to answer objections" - seems to stem from a similar context.
