Luke [chapter 9:28-36] relates the Transfiguration to the mission of Jesus as he moves towards death and glory. Jesus is praying, and the light shines on his face. We do not know that it is a prayer of agony and conflict like the prayer in Gethsemane, but we know that it is a prayer near to the radiance of God and the prayer of one who has chosen the way of death. Luke tells us that the two witnesses were conversing about the exodus which Jesus would accomplish in Jerusalem: not the death alone, but the passing through death to glory. the whole going forth of Jesus as well as the leading forth of the new people of God in the freedom of the new covenant. Luke tells us that after the resurrection Jesus spoke of the witness of Moses and of all the prophets to his suffering and glory.
It was not a glory which the disciples at the time could fathom. No doubt they would have welcomed a glory on the mountain far away from the conflicts which had happened and the conflicts which were going to happen as Jesus set his face towards Jerusalem. Yet when Jesus went up the mountain to be transfigured he did not leave these conflicts behind, but rather carried them up the mountain so they could be transfigured with him. It was the transfiguration of the whole Christ, from his first obedience in childhood right through to the final obedience of Gethsemane and Calvary.
The disciples could not grasp this at the time, but the writings of the Apostolic age were to show that the link between the suffering and the glory came to be understood as belonging to the heart of the Christian message.
[Michael Ramsey, Be Still and Know]