"He commanded them that they should tell no one the things they had seen, till the Son of Man had risen from the dead" (Mark 9:9)
As the disciples were commanded, you should also say nothing until the Son of Man has risen in you - until the life of the risen Christ so dominates you that you truly understand what He taught while here on earth. When you grow and develop the right condition inwardly, the words Jesus spoke become so clear that you are amazed you did not grasp them before. In fact, you were not able to understand them before because you had not yet developed the proper spiritual condition to deal with them.
Our Lord doesn't hide these things from us, but we are not prepared to receive them until we are in the right condition in our spiritual life. Jesus said, "I still have many things to say to you, but you cannot bear them now" (John 16:12). We must have a oneness with His risen life before we are prepared to bear any particular truth from Him. Do we really know anything about the indwelling of the risen life of Jesus? The evidence that we do is that His Word is becoming understandable to us. God cannot reveal anything to us if we don't have His Spirit. And our own unyielding and headstrong opinions will effectively prevent God from revealing anything to us. But our insensible thinking will end immediately once His resurrection life has its way with us.
"... tell no one ...". But so many people do tell what they saw on the Mount of Transfiguration - their mountaintop experience. They have seen a vision and they testify to it, but there is no connection between what they say and how they live. Their lives don't add up because the Son of Man has not yet risen in them. How long will it be before His resurrection life is formed and evident in you and in me?