Here are some thoughts on Jesus’ transfiguration by Fred Craddock:
We would very much like to penetrate the mystery of this experience, but we cannot. Matthew calls it a vision. One thing is clear: Jesus and his three disciples have an experience of God. Its meaning for Jesus and for them is different, but the only actor in the event is God. Jesus is not acting but is being acted upon. The God of Moses and Elijah affirms them in their unity with Jesus but asserts the finality of Jesus. The God who could rescue the Son from suffering confirms for Jesus the way of the cross. This God also tells the disciples, who will soon face conditions that seem to derail if not bring to an end their hope in Jesus, that those very painful conditions do not lie across the way but on the way to the completion of God’s purpose. This is a mountaintop experience but not the kind about which people write glowingly of sunrises, soft breezes, warm friends, music, and quiet time. On this mountain the subject is death, and the frightening presence of God reduces those present to silence. In due time, after the resurrection, they will remember, understand, and not feel heavy. In fact, they will tell it broadly as good news.
From Luke (Interpretation Commentary)