Transfiguration

And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.  And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.  And there appeared unto them Elias with Moses: and they were talking with Jesus.
Mark 9:2-4

Jesus allowed only three disciples to witness the Transfiguration because once they are strengthened they will strengthen others. The three of them did not want to leave as they quickly thought of building shelters. Now you can imagine if all the disciples were present.

The disciples needed to see God's glory as Jesus transfigured on the mount. They had never seen this image before and they were not to tell anyone until after the resurrection.

Moses represented the Law and Elijah represented the Prophets. Both the Law and the Prophets point to Jesus. A voice confirmed the Son and that all should listen to him.

The disciples did not foget this encounter.  John testified, And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. John 1:14

Peter spoke about this experience, For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty.  For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased.  And this voice which came from heaven we heard, when we were with him in the holy mount.
2 Peter 1:16-18