And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.Matthew 17:9
And as they came down from the mountain, he charged them that they should tell no man what things they had seen, till the Son of man were risen from the dead.Mark 9:9
Once again, we have a scenario where something truly remarkable has happened and Jesus tells the witnesses to not say anything to others. However, this time Jesus puts a time constraint on their silence. I think this is the key to possibly understanding why the trio of disciples was instructed to keep the transfiguration a secret.
Some logical possibilities for the instruction could be that all the disciples were prone to doubt like Thomas after the resurrection. Jesus is saving them from the sin of doubt! It could also be that the other disciples might be jealous that these three got to see and hear more than they did. If that is the case, then Jesus is saving them from the sin of jealousy.
These may be possible but it is clear from the next verses that these three disciples still did not understand what Jesus meant by raising from the dead. They no doubt believed in the general resurrection at the end of time like most Jews did (except the Sadducees). But they did not yet understand the necessity of Jesus suffering, dying, and raising from the dead.
By having the disciples keep this experience a secret until after Jesus was resurrected saved these three and everyone else from confusion. After Jesus rose from the dead, then the scene on the mount of transfiguration is more easily understood.
The resurrection is the cornerstone of the gospel. It explains everything that was in shadow in the Old Testament, and gives power to our faith so that we can follow Christ!
The reason that Moses and Elijah were able to stand there in bodily form with Jesus is because Jesus took on flesh and died for fleshly men! Without the death of the Jesus, there is no resurrection of Jesus.
And Jesus answered them, saying, The hour is come, that the Son of man should be glorified. Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit.John 12:23-24
Without the resurrection of Jesus, there is no regeneration or resurrection for the children of God. But He is risen! And we have hope!
But if there be no resurrection of the dead, then is Christ not risen: And if Christ be not risen, then is our preaching vain, and your faith is also vain. Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up, if so be that the dead rise not. For if the dead rise not, then is not Christ raised: And if Christ be not raised, your faith is vain; ye are yet in your sins. Then they also which are fallen asleep in Christ are perished. If in this life only we have hope in Christ, we are of all men most miserable. But now is Christ risen from the dead, and become the firstfruits of them that slept.1st Corinthians 15:13-20