NOTES for Pe2 1:17-18
As seen, for Peter (as, indeed, and for the whole Church) the transfiguration of Jesus on the mount Tabor is a witness of His greatness. What really happened there? There is no doubt that the apostles saw their Master on Tabor in a transfigured body, in a body belonging to the Kingdom according to its nature, in the same body, in which they later saw Him resuscitated. In what was the sense of the vision? And what preceded it in the yahvism and in the Jewish tradition? It is completely obvious that in this case, it is about the revelation of the Kingdom. Such revelations were known since the Antiquity. The visions of the Throne of glory became a part of the liturgical tradition already at the time of Salomon, because they were nothing else, than the revelation of the Kingdom, which of course "is not close" yet, but which the reflections all the same sometimes penetrated into the non transformed world. It was sometimes revealed otherwise, in particular, in the visions of the celestial chariot of Yahweh that Ezekiel for example saw: this chariot carried the same Throne of glory, as the others (for example, Isaiah of Jerusalem) saw in the Temple. And here is now the place of the vision of the Throne of glory took the vision of the transfigured Savior. It was of course as well the revelation of the Kingdom, and the apostles understood everything perfectly.
They did not understand only one thing: in order that the Kingdom enters the world definitively, the Messiah had to pass His way to the end, and this end should not be the victory in the messianic war, but the death on the cross and after the resurrection. But the fact that Jesus appeared before the apostles like that, became for them a convincing witness of His messiahship: because the Kingdom belongs in particular to the Messiah, and so now their Master appears before them, as a King (what was obvious), this means that He is really the Messiah, and the Kingdom really belongs to Him.
It is not accidentally if Jesus Himself especially begins to speak a lot and often to His apostles about His death and His resurrection in particular after the vision on the mount Tabor: for, now they already knew almost everything, and they had to understand the main thing: how the Kingdom will enter the world. But the main thing, as seen, turned out to be the most difficult, and especially because it contradicted all the representations accepted then about the Messiah and the Kingdom. But, as seen, the misunderstandings after the Pentecost were dispelled, but the memory of a remarkable event remained. And was kept by the Church.