As Peter, James, and John ascended the mountain with Jesus in Mark 9, they had no idea what they were about to see. The transfiguration of Christ terrified them, but it also manifested an extremely important truth. This truth is echoed by Paul in 2 Corinthians 3:7-11 when he speaks of the fading glory of the law. According to Paul, the Ten Commandments and all they represent have glory yet this law is a ministry of death which is fully overshadowed by the glory of the grace of Christ. Evidence that Peter was later given understanding of what he witnessed on the mountain with Jesus is seen in Acts 3. In this passage Peter explains Christ is the one of whom Moses spoke when he said a prophet would come in the future to whom we all should listen.
In the transfiguration the glory of Christ shines bright. Peter was not given time to build three tents as he suggested because at the time he did not understand what he was seeing. The attempt to build three tents ignored the exalted state of Christ and His grace over Moses and the law. Peter did not yet understand. In verse 9 the three disciples are told to tell no one what they had seen until the Son of Man had risen from the dead. We are not given the reason for this, but it may be partly due to the fact that they did not understand what they had seen.
We do know that Peter later understood the purpose of this event after the Holy Spirit came upon him. He proclaimed truth about the fading glory of the law after the resurrection of Christ. He exalted the grace of Christ fully above the requirements of the law. What he was given a glimpse of in Mark 9 was now fully known and accomplished in the death and resurrection of Christ. In this truth we rest. Christ is now seated at the right hand of God the Father almighty and through the grace of Christ we no longer fear the condemnation of the law.