In 'Binding the Strong Man' [Orbis, '88, '97, p.250] Ched Myers speaks of what happens on the mountain top as 'a kind of salvation-history summit conference'. Not bad?!
We all relate to Moses as 'representing' the Law. (Imagine him knocking at your front door: "Hello there! I represent the Law." & we reply, "What do you mean? Show us your ID!") What does it mean that Moses represents the Law? The same goes for Elijah; what does it mean that he represents the Prophets? As Christians see it, Jesus is the new & greater Moses & Elijah, & therefore newer than & greater than whatever it is they 'represent'. Their job's done now. Time to pass the baton & fade away. Forever important, but no longer controlling figures in humanity's spiritual journey.
LK tells us (9:31) that Moses & Elijah speak with Jesus about his 'exodus which he was about to accomplish in Jerusalem', reminding us: a) that after initial panic (who could blame him?) Moses led the great Exodus through the Sea of Reeds; b) also panicking, Elijah flees an untimely exodus at Ahab's hands till Yahweh presses him back into service to finish his job. On the mountain top Jesus by-passes M & E's panic reactions to what lies ahead, & re-commits to being the Servant who will carry his cross to his own Exodus. From death to resurrected life beyond. His glowing with divine light up on the mountain is only a momentary interlude in his journey as the human face of God to a shocking death. it is part of God's pattern that Moses & Elijah both die in unusual circumstances, but for God's Son to die on a cross...........defies belief.
Except that they're spectators at something really special, the disciples are as puzzled as usual at what's going on. Maybe Peter's wanting to build shelters is a 'type', a forerunner of the way we construct church comfort zones by wanting to pin God-things down, keep them as they are, bring them under any little control we can manage. Which is exactly what Jesus eschews (I can't find a better word than that old one) on the mountain top. If there's a case for anyone to stay up there, it's Jesus! But he has an Exodus to accomplish in Jerusalem. So down the mountain he comes. We know what he runs slap bang into! People wanting to come in from their margins.
Coming down the mountain, Jesus tells the disciples (as often in MK) to keep to themselves what they've seen 'except when the Son of Man (etc.) should rise from the dead'. We need to hold in tension, as Jesus himself obviously had to do, both 'thrusts' implicit in 'Son of Man' - (human) Suffering Servant, & (divine) Saviour. Is it when the going gets too hard fulfilling our humanity through serving each other, inevitably suffering something in the process, is it then some of us lean more in the direction of supernatural intervention? When is it time to head up our mountain? When is it time to come resolutely down?