The place to begin of course is to notice the dates that Alexander Roberts and James Donaldson give for the life of Gregory - 205 - 265 CE. Here is how they reconstruct the life of 'Theodore' (= Gregory):
Our Gregory has given us not a little of his personal adventures in his panegyric upon his master, and for his further history the reader need only be referred to what follows. But I am anxious to supply the dates, which are too loosely left to conjecture. As he was ordained a bishop “very young,” according to Eusebius, I suppose he must have been far enough under fifty, the age prescribed by the “Apostolic Canons” (so called), though probably not younger than thirty, the earliest canonical limit for the ordination of a presbyter. If we decide upon five and thirty, as a mean reckoning, we may with some confidence set his birth at A.D. 205, dating back from his episcopate, which began A.D. 240. He was a native of Neo-Cæsarea, the chief city of Pontus, — a fact that should modify what we have learned about Pontus from Tertullian.
He was born of heathen parentage, and lived like other Gentile boys until his fourteenth year (circa A.D. 218), with the disadvantage of being more than ordinarily imbued by a mistaken father in the polytheism of Greece. At this period his father died; but his mother, carrying out the wishes of her husband, seems to have been not less zealous in furthering his education according to her pagan ideas. He was, evidently, the inheritor of moderate wealth; and, with his brother Athenodorus, he was placed under an accomplished teacher of grammar and rhetoric, from whom also he acquired a considerable knowledge of the Latin tongue. He was persuaded by the same master to use this accomplishment in acquiring some knowledge of the Roman laws. This is a very important point in his biography, and it brings us to an epoch in Christian history too little noted by any writer. I shall return to it very soon.
I will cut off Schaff's reconstruction of the life of Gregory at this point because he accepts a lot of the legendary details of the life of Gregory which are rejected by modern scholarship and moreover which contradicts what is explicitly stated in the Panegyric for Origen - i.e. that this fourteen year old boy was initiated by Origen after a seven year initiation.
Of course the major difficulty for this theory is that most scholars think that Origen only became a permanent resident of Caesarea c. 231 CE. Lee I. Levine's Caesarea Under Rome Rule provides us with a typical reconstruction of events from the Church History of Eusebius:
An especially close relationship apparently existed between Palestine and Alexandria. Noting that the Alexandrian church celebrated Passover on the same day indicates the prestige of the latter church in Palestinian circles. In addition, the constant exchange of letters between these two countries attests to their close relations. Origen's fleeing to the city [of Caesarea] from Alexandria in 215-216, his return visit in 231-232 and subsequent residence there may have been a result, in part, of these ties, and in turn undoubtedly strengthened them. [p. 116]
As I noted previously, Levine's reconstruction of the movement of Origen is based on the perplexing narrative of Eusebius which we are stuck with. Yet I wonder if the 'Theodore' of the Mar Saba letter can be tentatively identified with Gregory Thaumaturgus once we take a second look at the actual evidence again from Eusebius and see the passage which confused previous generations of scholars into thinking that Origen left Caesarea shortly after arriving in Caesarea in 216 CE.
I have already shown this to my readership once before but it is worth revisiting again. Eusebius devotes two chapters in Book Six to Clement of Alexandria. Clement is clearly the subject of chapters 12, 13 and 14. At the end of chapter 11 there is a reference to a 'Adamantius' which Eusebius says 'was also the name of Origen' and which countless generations of scholars have taken to mean Origen himself (i.e. Origen was also named Adamantius). I have always wondered if Clement is still the subject of the continuous narrative and Eusebius is saying that both Clement and Origen were given this name (which ultimately derives from Ezekiel's description of Adam in Paradise i.e. they regained the 'Adamantine' state of Adam):
To sum up briefly, he [Clement] has given in the Hypotyposes abridged accounts of all canonical Scripture, not omitting the disputed books, — I refer to Jude and the other Catholic epistles, and Barnabas and the so-called Apocalypse of Peter. He says that the Epistle to the Hebrews is the work of Paul, and that it was written to the Hebrews in the Hebrew language; but that Luke translated it carefully and published it for the Greeks, and hence the same style of expression is found in this epistle and in the Acts.
But he says that the words, Paul the Apostle, were probably not prefixed, because, in sending it to the Hebrews, who were prejudiced and suspicious of him, he wisely did not wish to repel them at the very beginning by giving his name. Farther on he says: But now, as the blessed presbyter said, since the Lord being the apostle of the Almighty, was sent to the Hebrews, Paul, as sent to the Gentiles, on account of his modesty did not subscribe himself an apostle of the Hebrews, through respect for the Lord, and because being a herald and apostle of the Gentiles he wrote to the Hebrews out of his superabundance.
Again, in the same books, Clement gives the tradition of the earliest presbyters, as to the order of the Gospels, in the following manner:
The Gospels containing the genealogies, he says, were written first. The Gospel according to Mark had this occasion. As Peter had preached the Word publicly at Rome, and declared the Gospel by the Spirit, many who were present requested that Mark, who had followed him for a long time and remembered his sayings, should write them out. And having composed the Gospel he gave it to those who had requested it.
When Peter learned of this, he neither directly forbade nor encouraged it. But, last of all, John, perceiving that the external facts had been made plain in the Gospel, being urged by his friends, and inspired by the Spirit, composed a spiritual Gospel. This is the account of Clement.
Again the above-mentioned Alexander [of Jerusalem], in a certain letter to Origen, refers to Clement, and at the same time to Pantænus, as being among his familiar acquaintances. He writes as follows:
For this, as you know, was the will of God, that the ancestral friendship existing between us should remain unshaken; nay, rather should be warmer and stronger. For we know well those blessed fathers who have trodden the way before us, with whom we shall soon be; Pantænus, the truly blessed man and master, and the holy Clement, my master and benefactor, and if there is any other like them, through whom I became acquainted with you, the best in everything, my master and brother.
So much for these matters. But Adamantius, — for this also was a name of Origen (ὁ γέ τοι Ἀδαμάντιος καὶ τοῦτο γὰρ ἦν τῷ Ὠριγένει ὄνομα) —when Zephyrinus was bishop of Rome, visited Rome, desiring, as he himself somewhere says, to see the most ancient church of Rome.
After a short stay there he [i.e. Clement] returned to Alexandria. And he performed the duties of catechetical instruction there with great zeal; Demetrius, who was bishop there at that time, urging and even entreating him to work diligently for the benefit of the brethren.
I wonder whether Eusebius has been using a source which knew that Clement was the first to be called 'Adamantinus' (the 'Adamantius' at the heart of the anti-Marcionite treatise De recta in deum fide was another). In other words, it was Clement - not Origen who went to Rome to see Zephyrinus, and then returned to Alexandria. For it is harder to believe that Origen and Demetrius behaved like a hot-blooded couple (i.e. always fighting and reconciling). The Alexandrian tradition prided itself in its impassability.
To this end, it was Clement who fled Alexandria in the Commodian period (or perhaps the beginning of the Severine age) only to reside with Alexander of Jerusalem and a number of Alexandrian ex-pats until 215 CE. It is at this point that Clement (a.k.a. 'Adamantius') went on to Rome and then returned to Alexandria. Origen by contrast stayed in Alexandria until the time Clement left and then made Caesarea his more or less permanent home. 'Theodore,' according to this reconstruction, was Gregory Thaumaturgus who came to Caesarea while Origen was at home there and Clement had been relocated to Alexandria.
At some point during his instruction by Origen (perhaps 219 CE), Gregory (a.k.a. 'Theodore') sent Clement a letter asking about the 'mystic gospel.' Clement, in one of his last letters, answered Gregory's questions with the information provided in the surviving manuscript which may have been truncated because Origen is ultimately referenced. This isn't as crazy as it might sound at first. It would help explain why the letter was separated from the greater body of 'letters of Clement of Alexandria' existing in the Mar Saba monastery as late as the eighth century.
The letter is as much a letter to the ever popular Gregory as it is to the obscure Clement of Alexandria. It is worth considering ...