Mark 9:1-13
According to the promise in v. 1, three disciples are now to be allowed a preview of "the kingdom of God come with power". This kingdom is set forth in the King Himself, whom they recognize as the Lord Jesus, their Master, clothed in majesty and in resplendent glory. The One who usually veiled His glory and hid it beneath the humble "form of a servant", here uncovers it for a moment to the view of His dazzled and amazed disciples (Psalms 104:1). Then a voice comes from the cloud; it is for us also: "This is my beloved Son; hear him." The more great and dignified a person is, the more important are his words. Now the person we are invited to hear is none other than the well-beloved Son of God. Let us pay even greater attention to His teaching (Hebrews 12:25).
However good it was to be on the mountain (v. 5) they must come down from it and the Lord makes the three disciples understand that what they have seen will only be fulfilled later. Neither John (represented by Elias, v. 13) nor He Himself has been received. That is why it is necessary now for Him to go to the cross and to suffer so much before entering into His glory.