After even Peter has a hard time accepting Jesus' predictions of His suffering and death, He takes them up a mountain and pulls back the veil of His true glory a little, for a minute. Elijah and Moses appear, speaking with Jesus, Luke says about His death. This convinces the disciples, but they wonder when Elijah will come. Jesus says He already did (John the Baptizer).
They come down the mountain, and like Moses the people are amazed on seeing Jesus. He heals a boy, casting out a demon. Moses and Jesus' disciples couldn't do this. The uniqueness of Jesus is clear!
Sins this may reveal
Our silliness when the glory of Jesus is shown us (vs 5-6).
Our slowness to understand what God is doing (vs 10-13).
All the caveats and conditions we put on Jesus' power to save and to heal (vs 23-24).