I'd like to bring Kevin Rosero's comments on my recent post on 2Peter and the Gospels to my reader's attention. Kevin offers some helpful criticisms which I hope to address here, rather than in the comments section where they might escape the reader's notice.
I must confess that I am not sure why Kevin suggests that this is the type of tradition that might remain private--what would inspire them to keep a lid on a tradition that Jesus had offered an accurate prophecy? I see no hint elsewhere of such tendencies, so must plead ignorance as to the basis of this proposal.
Kevin notes that John, unlike Mark and Matthew, is not sympathetic to Peter, and thus his inclusion of this prophecy belies my suggestion that we should expect to find it in pro-Peter material. I had anticipated this objection, but had hoped (albeit incorrectly!) that it would escape notice until I could access my books, and provide some support for the notion that John 21 is interpolated. That said, I hope it is of sufficient acceptance (a very healthy majority, to my understanding) so as not to inspire objection among my readership if I suggest it to be the case without sources: The author of Jn.21 is not the same as the author of the remainder of the text (I'd actually push that back to before the Doubting Thomas pericope, but that's not necessary in this context).
John is not generally sympathetic to Peter, preferring to throw his text behind the anonymous and enigmatic "Beloved Disciple." Can the same be said of the author of Jn.21? Despite John's efforts to the contrary (eg.20.4), the primacy of the disciple whom Jesus loved is negated: It is Peter Jesus asks to "tend my sheep," and the beloved disciple bears witness to this. This isn't to say that the Beloved Disciple is negated outright (he does, after all, recognize Jesus first), but he is clearly trumped by Peter in 21.15-18.
This brings us to another indicator of Johannine Redaction. A patently accurate prediction is put on the mouth of Jesus, immediately preceding a seeming apologia for an inaccurate prophecy (whether that prophecy goes back to Jesus, or is a tradition brought about by the Johannine "community" isn't pertinent here--what is pertinent is that it was attributed to Jesus). It seems reasonable to suggest that pseudo-John's effort is to cushion the blow somewhat. To indicate that Jesus always knew exactly what he was talking about.
I think Kevin may be misapplying Occam's Razor in his suggestion that direct knowledge of the GJohn is an uncessary entity. 2Peter's knowledge of some earlier source is absolutely necessary--1:14 makes no sense without such knowledge. The question is whether that source is John or something else, and something else is the extra entity, unless it can be shown to reasonably exist. That doesn't mean that it's impossible that it exists, rather, that on the basis of the current evidence, there is no reason to suggest that it does. I'd suggest that pointing out the Greek word "delow" and paraphrasing a lexicon is insufficient grounds for that.
Regarding the possibility of an independent tradition of the Transfiguration, Kevin notes that Luke has changed Mark's six days to eight, and suggests that this looks like the artifact of a separate source. Firstly, I think it bears noting that it is not unreasonable to suggest that Mark's "six days" had some significance to his readership (days of creation, perhaps?). It is rather unlike Mark to give so specific a time period, and not one to waste words, it seems to imply he is up to something. Luke's changing of the number of days could represent either his ignorance or rejection of that symbolism. I think this is insufficient as a way to establishing an independent tradition--to do so it would need to be illustrated that 8 days was both un-Markan and un-Lukan, or at the very least sufficiently "un-Lukan" to raise eyebrows.
The change in wording (followed by both Matthew and Luke) need not represent a shared, earlier tradition. In fact, I'd suggest the evidence indicates quite the contrary: Mark is creating a deliberate parallel to his own gospel in the transfiguration, one foreshadowed by Peter's earlier proclamation of Jesus as the Messiah. The earlier declaration of Jesus as God's son (Mk.1.11) is here confirmed. Matthew and Luke both seek to underline this--to ensure that the parallel is recognized for what it is, lest the less than astute reader miss it. One does not need to suggest (though I do endorse) that Luke knew Matthew for this to occur--it's a fairly recognizable parallel, and one that we might expect both to find on their own.
I'd suggest the evidence of Markan literary device in the words of God during the transfiguration is perhaps the strongest argument in favor of Markan authorship. The parallel to the baptism is quite significant in the Markan narrative as it signifies the inclusion of the disciples in the "Messianic Secret." If the words already existed as such, it is quite serendipitous for Mark. I'd suggest too serendipitous to be plausible, which brings us back to where we began: Mark made these words up, Matthew and Luke redacted them, underlining what Mark implies. 2Peter knows this. If not from the gospel, then how?
Thanks to Kevin for his kind words and thoughtful criticisms.