As with anything that Jesus did, there are many ways to look at the Transfiguration. Jesus was shown to be the Son of God to Peter, James, and John. However, I heard another way to look at it that stuck with me. Jesus was talking with, supposedly, dead people in Elijah and Moses. He was talking to saints.
When we as Catholics pray to saints, many protestants take offense to it. Some thinking that we are treating the saints as if they are gods themselves. Which we do not; we ask them to go to Jesus on our behalf. Some think we are talking to dead people that cannot possibly help us. Surely, Jesus did not talk to Elijah and Moses just for the fun of it. Everything that He did had a purpose.
As to the saints being “dead” people, we rely on what Jesus said to the Sadducees. “As for the dead being raised, have you not read in the Book of Moses, in the passage about the bush, how God told him, ‘I am the God of Abraham, [the] God of Isaac, and [the] God of Jacob’? He is not God of the dead but of the living. You are greatly misled.” – [Mark 12:26-27]
So, the saints are not dead. If they indeed are in Heaven, then they are more alive than we are. They have reached a fullness and relationship with God that we hope to experience. Indeed, when we ask for their assistance, we are asking our friends in ‘high places’ to ask God on our behalf. It’s good to have friends in ‘high places’, but it does us little good if we do not ask for them to help us.