There are many Patristic references which point to an additional Marcionite gospel (or indeed 'pro-evangelium') was in fact 'according to Mark' (Irenaeus AH 3.11.7, 9; Philosophumena 7.18). But there are other important parallels. The Marcionites shielded the identity of their evangelist (Tertullian Against Marcion 4.2; Adamantius 1.5) in the same way as Clement avoids identifying his scriptural sources and more importantly openly acknowledges that one should lie to protect Mark's authorship of the 'mystic gospel' (to Theod. II.10 - 12).
Yet there is another scriptural allusion in to Theodore which no one seems to have picked up and it only comes to the forefront when you have a familiar with obscure anti-Marcionite references:
Thus he composed a more spiritual Gospel for the use of those who were being perfected. Nevertheless, he yet did not divulge the things not to be uttered (απόρρητα), nor did he write down the hierophantic teaching of the Lord, but to the stories already written he added yet others and, moreover, brought in certain sayings of which he knew the interpretation would, as a mystagogue, lead the hearers into the innermost sanctuary of that truth hidden by seven veils [to Theod. I.21 - II.2]
The material that is embolded here clearly seems to represent a response to something that Theodore has told, undoubtedly that he had heard that Mark not only divulged but also wrote down an 'unspeakable' revelation that came to him from heaven.
It is in my mind very difficult not to connect this with what numerous Church Fathers tell us about heretics associated with someone by the name of Mark who have claim that 'Paul' was really named 'Mark' and that he revealed to them through a gospel the 'unspeakable' revelation through 2 Corinthians 12.4 "this man was caught up into paradise, and heard unspeakable (αρρητα) words, which it is not lawful for a man to utter." So we read:
and the apostle says, "Unutterable are the words I heard," and Marcion, said: "Me, I heard them!" So is it the apostle who believed these unutterable words which he must have heard or Marcion who rejects them [Eznik Refutations, Marcion 3 French translation of the Armenian original - et l'apôtre dit : Inénarrables sont les paroles que j'ai entendues. Et Marcion dit : Moi, je les ai entendues. Or, est-ce l'apôtre, qui croit ces paroles inénarrables, (qu'il) faut écouter, ou Marcion qui, les rejetant, les met au néant?]I have pointed out time and again that Clement understands 2 Corinthians in the exact same way as the Marcionites did - i.e. that the apostle received a heavenly revelation which was 'unspeakable' but which ultimately formed the basis to the mystic gospel of Mark.
These which Paul says: "Words which must not be uttered by any man." However as with Marcion, since he is a man, the words should have been for him unspeakable, him who is indeed the worst kind of man of all. [ibid - voici (ce que) dit Paul : (Paroles) qu'il ne faut pas que l'homme parle. Or, (quant à Marcion, s'il est homme, pour lui indicibles étaient ces paroles ; lui qui, 'en effet, est homme, est pire que ; tous les hommes]
But surely if Luke, who always preached in company with Paul, and is called by him “the beloved,” and with him performed the work of an evangelist, and was entrusted to hand down to us a Gospel, learned nothing different from him (Paul), as has been pointed out from his words, how can these men [i.e. Marcionites], who were never attached to Paul, boast that they have learned hidden and unspeakable mysteries? [Irenaeus AH 3.14.1]
we allege the same against those [Marcionites] who do not recognise Paul as an apostle: that they should either reject the other words of the Gospel which we have come to know through Luke alone, and not make use of them; or else, if they do receive all these, they must necessarily admit also that testimony concerning Paul, when he tells us that the Lord spoke at first to him from heaven [ibid AH 3.15.1]
Some of [Mark'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 (αρρητα). 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 [ibid AH 1.13.6]
Besides the above, they adduce an unspeakable number of apocryphal and spurious writings, which they themselves have forged, to bewilder the minds of foolish men, and of such as are ignorant of the Scriptures of truth [AH 1.19.1]
although Paul was caught up as far as the third heaven, and when brought into paradise heard certain things there, yet these revelations cannot be thought to be such as would render him more qualified to teach another doctrine, since their very nature was such that they could not be communicated to any human being. But if that unknown revelation did leak out and become known to some one, and if any heresy affirms that it is a follower of that revelation, then either Paul is guilty of having betrayed his secret, or some one else must be shewn to have been subsequently caught up into paradise to whom permission was given to speak out what Paul was not allowed to whisper. [Tertullian Prescipt. 24]
There is good reason for Clement then to reject the claims of those who have told Theodore that Mark wrote the mystic gospel from the words he heard by heavenly revelation. It makes absolutely clear that the Alexandrian tradition to which Clement belonged was Marcionite. This aspect of the Marcionite doctrine was well known to second and third century witnesses and continued to get passed on to later Church Fathers like Eznik because it was a core belief of the tradition. Indeed it makes Mark a successor to Moses and a precursor to Mani and Mohammed.
That Clement secretly accepted the idea that the apostle received the 'mystic logos' from said 'unspeakable revelation' is plainly evident in a number of statements in Clement's acknowledged writings including:
That μύστην λόγον for Clement means 'written text' like gospel or Law is plainly evident in another passage from the Stromata "it is to those who know hebdomad and ogdoad of the the mystic word (i.e. the Law) that the blessed David delivers clearly, praise thus (Σαφῶς δὲ τὸν περὶ ἑβδομάδος τε καὶ ὀγδοάδος μυστικὸν λόγον τοῖς γιγνώσκουσι παραδίδωσιν ὁ μακάριος Δαβὶδ ὧδέ πως ψάλλων): "Our years were exercised like a spider. The days of our years in them are seventy years; but if in strength, eighty years. And that will be to reign." [Str. 6.17] Clement ultimately appropriates the concept from Philo where it means 'Law of Moses.'
The point of course is that we have to learn to show Clement's utterances in the Letter to Theodore the appropriate sensitivity. He is clearly walking on hallowed ground. This is the very beating heart of Alexandrian Christianity - the explanation of how its patron saint received the divine gospel through heavenly revelation. He already acknowledges that he is capable of misinformation to protect the sacredness of that understanding. We are ultimately witnessing the convergence of what is called 'Marcionitism' in the hostile writings of the Church Fathers with the origins of Alexandrian Christianity.
To these statements [regarding the highest God being utterly unknowable] the apostle will testify: "I know a man in Christ, caught up into the third heaven, and thence into Paradise, who heard αρρητος words which it is not lawful for a man to speak," -- intimating thus the impossibility of expressing God, and indicating that what is divine is unutterable by human power; if, indeed, he begins to speak above the third heaven, as it is lawful to initiate the elect souls in the mysteries there ... And was it not this which the prophet meant, when he ordered unleavened cakes to be made, intimating that the truly sacred mystic law (μύστην λόγον) respecting the unbegotten (God) and His powers, ought to be concealed? In confirmation of these things, in the Epistle to the Corinthians the apostle plainly says: "Howbeit we speak wisdom among those who are perfect, but not the wisdom of this world, or of the princes of this world, that come to nought. But we speak the secret wisdom of God in a mystery." And again in another place he says: "To the acknowledgment of the mystery of God in Christ, in whom are hid all the treasures of wisdom and knowledge." These things the Saviour Himself seals when He says: "To you it is given to know the mysteries of the kingdom of heaven." And again the Gospel says that the Saviour spake to the apostles the word in a mystery. For prophecy says of Him: "He will open His mouth in parables, and will utter things kept secret from the foundation of the world." And now, by the parable of the leaven, the Lord shows concealment; for He says, "The kingdom of heaven is like leaven, which a woman took and hid in three measures of meal, till the whole was leavened." For the tripartite soul is saved by obedience, through the spiritual power hidden in it by faith; or because the power of the word which is given to us, being strong and powerful, draws to itself secretly and invisibly every one who receives it, and keeps it within himself, and brings his whole system into unity [Strom. v.80.3]
dating back to conversations with his grandfather, Gaston Frank. "He said we represent one of the last descendants of the Frankist Jewish faith in the world," he muses. "I grew up thinking that our family was something like the Last of the Mohicans."