Not all the disciples were treated equally. At special times in his life Jesus selected his closest friends to be with him. Here we have Peter, James and John chosen as witnesses to the Transfiguration.
The Transfiguration places Jesus with two great figures in Jewish history. Moses the giver of the Law and Elijah the great prophet. There was a tradition that Elijah would return to herald the Messiah. Jesus suggests that this tradition came to pass in the person of John the Baptist.
Christians need to remember that Jesus was a Jew, part of that people’s history and tradition. He saw himself as coming primarily for the ‘lost sheep of Israel’. One reason he was not accepted by a lot of his own people was the common expectation that the Messiah would lead the people to independence from foreign domination. At the time of Jesus, the conquerors were the Romans.
Source: Jesuits in Britain