Let let's start with the question as to whether or not the Letter to Theodore is actually referencing a fire ritual. There can be no doubt that no reference to fire or water for that matter is made anywhere in the text. Yet the critical thing to keep our eye on is Clement's statement which precedes the citation of the material from Secret Mark that:
Mark, then, during Peter's stay in Rome wrote an account of the Lord's doings, not, however, declaring all of them, nor yet hinting at the things pertaining to the mysteries, but selecting what he thought most useful for increasing the faith of those who were being instructed. [to Theod. 1.15 - 18]
In other words, fire isn't specifically referenced in the Alexandrian gospel. It is clearly a 'mystery of divine kingship' and thus necessarily involved a throne. But I think it quite likely that the ritual which was performed on the initiate involved his purification by fire.
We have already noted that Meyer points to the underlying similarities between Jesus instructing the rich youth to wait six days before entering the mysteries on the seventh and Exod 24:16 (Marvin Meyer Secret Gospels: essays on Thomas and the secret Gospel of Mark p. 124) Nevertheless Meyer ignored the fact that the Samaritans clearly understood Moses to have literally stood in fire while receiving the Torah:
He was making supplication during the six days and prostrating before the King of all kings; he saw the Sanctuary of the Unseen spread out in the fire within the cloud. He was called on the seventh day from the midst of the cloud and he saw the ranks of the angels in their array ... Fire glorified him seven times: fire's first dealing with him was on Mount Sinai at the beginning of his prophetic mission; it was revealed to him in the Bush; from it call was made by name twice— "Moses, Moses" (Ex. 3. 4)—a great wonder the like of which has never been in the world and never will be! From the fire he was called on the morning of the day of Horeb, from the top of Mount Sinai in the presence of six hundred thousand; fire flaming up to the heart of heaven, as he said, "While the mountain burned with fire to the heart of heaven" (Deut. 4. 11); then he trod in it with his feet and was not harmed by it; it was like a plant with the dew of heaven on it under his feet [Mimar Marqe 5.3]
The Samaritan tradition of Marqe and the Alexandrian text of Ezekiel the tragedian both confirm that Moses was being purified to sit in the divine throne. I am quite certain that the original gospel was arranged in much the same way - i.e. what is being described as a 'mystery' in SGM 1 was completed at the conclusion of the gospel. In other words, the disciple rather than Jesus is depicted as sitting on the divine throne in the final act. I will have more to say about this later.
It is enough to say for the moment that the Pentateuch has this twofold division. Exodus 24.16 is thus the preparation for the 'final act' in Moses's enthronement viz. Deut 33.2. As Meeks notes (the Prophet King p. 196) all references to Moses as king go back to Deut 33:2. Here Moses is the lawgiver who has transmitted the divine legislation of Torah to the people and leaves Joshua as his successor to watch over the people and the Law. According to tradition Moses ascended from Sinai to heaven where he was enthroned as king of Israel. When he returned he brought down with him the heavenly fire, the Torah, as the basis to his rule.
It might be useful to pay special attention to the wording of the passage in Deuteronomy and its enigmatic phrase - eshdat lamo. The King James version renders Deut 33:2:
The Lord came from Sinai, and rose up from Seir unto them; He shined forth from mount Paran, and He came with ten thousands of saints: from His right hand went a fiery law [eshdat] for them.
The word אֵשְׁדָּת (eshdat) appears only once in the Torah and is taken to mean here “the fire of Torah is with him ” or “fire of religion is with him.” The early Jewish interpretors used notrikon - a method where two words are combined into one or one word is divided into two - to explain this word eshdat. It was divided into two, the result being esh and dat - “fire and law" - this even though the word dat is Aramaic. The original word eshdat is Persian interestingly enough but no one knows what it exactly means.
The Torah that is described by the word eshdat was made out fire not stone. But how can one distinguish between the fiery tables and the fiery text? To solve the problem, the fire was thought to have been composed of two colors, black and white. The tablets were made out of black fire and the letters were made out white fire. The Law thus looked like a blackboard with white words chalked on it. In the words of the Talmud, “The white fire was engraved on black fire and it was a mixed fire and carved out of fire.” This was the magical image the Law possessed prior to Mount Sinai. And in case the skeptical reader is too quick to reject this interpretation out of hand, s/he should be reminded that the King James Version thought that it was appealing and translated this word accordingly.
The point of course is that the early Samaritan tradition of Marqe was very quick to connect everything associated with Moses's theophany with fire. Moses not only stands in the fire but clearly eventually becomes one with fire and is able to receive the 'fiery law.' It can't be coincidence then that Simon Magus - who is always said to be a Samaritan is connected with a deep interest in fire. It was a Samaritan obsession. Yet we should not overlook either that he and Mark were identified as Magians (= fire worshippers) too. It is amazing to see how many people overlook this repeated interest in fire. Both must have been connected with the Alexandrian fire baptism tradition.
Yet the key to begin to make sense that SGM 1 is to look carefully at the original Alexandrian episcopal throne and note that the eshdat limo theme was taken over by those who first constructed the relic. On each side of the corona of the throne there are a total of five torches which brings the total to exactly ten (see below from my personal photos of the relic). The idea must clearly be that not only was Mark like Moses (a point also preserved in the Samaritan tradition with respect to Marqe) but moreover that the one sitting in the chair is living Moses who has been also purified by fire.
The clearest example of the use of a mystical purification rite in ancient Alexandria in association with its Patriarchs is found in Severus Al'Ashmunein's History of the Coptic Patriarchs written around the end of the tenth century. Severus clearly preserves a much older tradition associated with the very divisive Patriarch Demetrius who was responsible for ultimately chasing Clement and Origen out of the city. There is always a sense in the literature that Demetrius was appointed to the throne of St. Mark from without. He is unlikely to have been a native Alexandrian and appears on the scene sharply at odds with the ascetic traditions of the Church.
Demetrius came to Alexandria a married man and immediately rouses the resentment of those who already thought him uncultured and unlearned. Before Demetrius had undergone the ritual fire purification contemporary "bewailed the fall" of the tradition of St. Mark. Yet we are told that on the very day of Pentecost, this Demetrius proved himself to worthy of the throne of St. Mark by being exposed to fire and not being burned:
So on the morrow, which was the feast of Pentecost, the patriarch celebrated the liturgy, and bade the archdeacon give directions to the clergy and the people that not one of them should leave the church, but that they should gather together round the patriarchal throne. The archdeacon, therefore, proclaimed to the congregation: "The patriarch's wish is to speak to you all. Let none of you, therefore, depart without hearing what he shall say." When they had sat down, the patriarch bid the brethren collect much fuel; and they did so, marvelling thereat and saying : "What is this that the patriarch will do?" Then he said to them : "Rise and let us pray!" So they prayed, and afterwards sat down. And he said to them : "I beg you out of your love for me, to allow my wife to be present before you, that she may receive of your blessing." Then they marvelled, and thought in their hearts : "What is this that he does?" And they all said : "Whatever thou biddest us do shall be done." Then the patriarch commanded one of his servants, saying : "Call my wife, the handmaid of the saints, that she may receive their blessing." So the holy woman entered, and stood in the midst of the congregation. And her husband, the patriarch, arose, where they could all behold him, and stood by the blazing logs, which had already been lighted, and spread out his cloak, and took burning embers from the fire with his hand and put them in his cloak; and all the spectators were astonished at the quantity of burning fuel in his garment, and yet it was not burned. [History of the Coptic Patriarchs 4]
The image here is clearly the same as that which we have noted accompanies the description of Moses reception of the Law in Samaritan writings - i.e. that he could stand in fire and not be burned. Yet the fact that this Alexandrian fire purification demonstration occurs on 'Pentecost' (= Shavuot) is absolutely significant as well. For this is the tradition celebration of Moses's reception of the ten commandments which as we noted was done through fire.
The point of all of this is the fact that the Alexandrian fire purification/enthronement ritual likely occurred on Pentecost (which Acts 2:3 also associates with 'fire') does not affect our theory about the original significance of SGM 1 in any way. This is because what is described in Secret Mark is only reporting Jesus's instruction of the youth into the 'mysteries of divine kingship' took place at the beginning of the first month of the Hebrew calendar. The Diatessaron, the Coptic Acts of Peter and a number of other sources all point to the Transfiguration narrative (= the final enthronement which confirmed the resurrected Christ) actually occurred after the Passion narrative. In the Acts of Peter it specifically occurs on Pentecost. Yet even without such radical reinterpretation of the narrative, even as it stands now the gospel ends with a heavenly enthronement on Pentecost.
More on all of this later ...
BTW - if SGM 1 occurred on the first day of the first month (the date fire is kindled in the Persian tradition) then Shavuot (which always occurs on the eighth day = Sunday of the eight week in the Samaritan tradition) would also theoretically be 64 days away (= 8 x 8). The Coptic calendar still calls the first day of the year Nayrouz (= Persian 'Nowruz' i.e. the day fire is kindled to start the year). The date is a purely Christian adoption; it was not so named in the pagan period.