The transfiguration was the final revelation and confirmation of the identity and divine authority of Jesus. He was man yes, but He was also fully God. From the start of chapter 9 Luke gives greater evidence that Jesus was indeed Christ, and is brought to a climax with Peter’s confession, Jesus foretelling and instruction, and finally the Transfiguration. It was a revelation designed only for His inner circle of disciples, namely Peter, John and James.
It could also perhaps have been to comfort them after His sudden foretelling of His death. That He who was to “suffer many things and be rejected by the elders and chief priests and scribes, and be killed, and on the third day be raised” was He who was glorious, amazing, astounding, beautiful, a sight and being to behold. They had nothing to fear.
Yet Peter, who saw the Transfiguration, denied Christ. Who am I to think I would escape such earthly fears? If Peter, who dwelt with Christ in intimacy, privy to His private teachings and counsel, could deny Him – what might I do when the rubber hits the road? Oh help me to love You, love You, love You.