Mark 9 (New International Version)
2 After six days Jesus took Peter, James and John with him and led them up a high mountain, where they were all alone. There he was transfigured before them. 3 His clothes became dazzling white, whiter than anyone in the world could bleach them. 4 And there appeared before them Elijah and Moses, who were talking with Jesus.
5 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." 6(He did not know what to say, they were so frightened.)
7 Then a cloud appeared and enveloped them, and a voice came from the cloud: "This is my Son, whom I love. Listen to him!"
8 Suddenly, when they looked around, they no longer saw anyone with them except Jesus.
In this account, two Covenants of God are represented. The Old Covenant, was represented by the law and the prophets (Moses and Elijah respectively). Jesus represents the New Covenant.
Also, Peter, James and John were the first witnesses to the revelation by God that the New Covenant will be coming soon (as Jesus has not yet been crucified) and that they would become partakers of the New Covenant after Jesus’ death.
Peter, suggested making three tents, one for Elijah, Moses and Jesus, probably because Peter wanted to stay and fellowship with the three. However, God interrupted Peter with a, thick cloud cover of Jesus, Moses and Elijah, with the words "This is My beloved Son whom I love. Hear Him!"
After this Moses and Elijah both disappeared and only the Lord Jesus was left with them.
How amazing the revelation of God to Peter, James and John that they are to become followers of the New Covenant. Peter probably wanted fellowship with the Old and New Covenants together (making the three tents) but God declared that we listen to and obey Jesus.
Only Jesus was left with them, what a lucid revelation that we listen to Jesus and obey Him. Jesus is the fulfillment of the law and the prophets, only Jesus stays – this is new covenant believing. Rejoice.