There are two places in the Gospels where the disciples experience the full glory of Jesus. This is one; the other is the resurrection.
Jesus, on the mountain, like Moses in Exodus 24, encounters the glory of the father. The people cannot bear to witness Moses' face so he wears a veil. But the disciples look on the face of Jesus during the encounter and after. Jesus has revealed the power and presence of God in the flesh, something Moses could not do. Yet here we find Moses again!
Some questions that arise from this passage:
1. How did Peter know it was Moses and Elijah? Jews did not carve statues of their leaders nor did they paint pictures of him. The only thing he could base their identity on was the prophecies.
2. Why did Peter want to build tabernacles for the three? Did he think he could keep them right there forever?
3. Why did Jesus only let three of his disciples view this and not the 12?
4. Why were the disciples afraid when the voice said This is my beloved son. Was it particularly loud? How did they know it was God's voice? Did God's voice sound anything like Jesus' voice?
5. Why did Jesus tell them not to tell anyone about this incident?