"And there appeared before them Elijah and Moses, who were talking with Jesus." NIV translation
Have you ever wondered why Moses and Elijah appeared with Jesus? Elijah represented all of the prophets while Moses represented the Law. The purpose of both the prophets and the Law were to point people to the need for Jesus as their Savior. The Law has never saved anyone and the words of men have no power to save. It is only through faith in the work of Jesus that we can be saved. We must remember that the disciples that were looking on were all Jews and would have known about the Law and the prophets.
"Peter said to Jesus, 'Rabbi, it is good for us to be here. Let us put up three shelters - one for you, one for Moses and one for Elijah.' (He did not know what to say, they were so frightened.)" NIV translation
Peter was well-known for saying things just off the top of his head and he did in this case as well. Isn't it good for us to see that these apostles were still men with quirks like the rest of us?
"Then a cloud appeared and enveloped them, and a voice came from the cloud: 'This is my Son, whom I love. Listen to him!'" NIV translation
A cloud appeared and God spoke from it because mortal men cannot see the face of God and live. The Father acknowledged Jesus as His Son and told the apostles to listen to Him. The message was clear as the Father was telling them that Jesus is the only way to be saved and not the Law or the words of a man. This teaching would be crucial in the early life of the church as men would have to resist their desire to add works to grace and to follow the latest teachings of a so-called prophet.
It is just as important for us today as it seems that every little denomination holds onto a piece of the Law and hold it up as a sort of test of salvation. In the same way, we are told that, in the last days, man will follow all kinds of false prophets. We have the power to overcome this through the Holy Spirit that resides in us.
"Suddenly, when they looked around, they no longer saw anyone with them except Jesus." NIV translation
The Law (represented by Moses) and the prophets (represented by Elijah) disappeared and all that was left was Jesus. This is a reminder to us that He is the only way to salvation.