AFTER the exaltation of seeing Jesus transfigured before them and seeing his glory, the disciples had to come down to earth again, and face the perils and sorrows and weaknesses of this earthly life. So is it always during our earthly pilgrimage. There are times when we are exalted by spiritual experience from the Holy Ghost, but while we are in this life, they can only be brief, and then the struggle of life must continue.
However notice that they came down with Jesus. Jesus was with them. Not only was he with them but he was instructing them. Jesus promised never to leave or forsake his disciples during their time on earth, and he keeps his promise. He has sent the Comforter to be with us always. He is one like Christ and he teaches, leads, strengthens us, and exalts Jesus to us. By the Holy Spirit we have the conscious presence of Jesus with us, and he is there to instruct us how we live and serve him. So we can face the trials of walking the narrow way.
The instruction as the disciples came down the mountain was concerning how they should react in their immediate life after what they had seen. Times of great spiritual uplift, when heavenly things are very near, and we see and know the presence of Christ more deeply than usual, are not always easy to handle, and we need instruction as to how we react to them in our ongoing life. Paul had an experience of being lifted into the third heaven (2 Corinthians 12). He could have boasted about this and been proud, but this would have been wrong. He needed to be instructed as to how he should relate to the experience and how he should go on with his life in the light of it.
The disciples here were instructed not to tell anyone about what they had seen of Christ's glory until after he had risen from the dead. Such instruction seems hard to understand, but plainly it was not appropriate or wise for this exalted knowledge of Jesus which Peter, James and John had received to be openly declared at this time. No doubt such a revelation could cause a stir which would make the execution of his saving work so much harder.
What the disciples had been granted was concrete assurance of the true nature of Jesus. They saw him in his true glory. They were given by the testimony of God the Father, that Jesus was divine and the eternal Son of God, the promised Christ. It was a stupendous and glorious blessing that they had been given. After the resurrection this testimony which was then given them fitted in with the victory of the resurrection, and gave this added and powerful assurance of who Jesus is, and of the glory of what he has achieved.
Having been given this revelation that Jesus was the promised Christ and Messiah, questions rose up in the disciples minds. They had been taught that before the Christ came, Elijah would return, and this would be a sign that the coming of Christ was imminent. The disciples had difficulty with the revelation they had seen because Elijah had not come. The problem for them, as it was for all Jews at the time was that the teaching of the Jewish teachers was that they taught that Elijah would come in his physical presence, and this had not happened.
In answering this dilemma Jesus, first of all, says that the coming of Elijah before the coming of the Christ was essential, and it was right to teach and believe this. He does this to indicate that the understanding as to how Elijah would come had not been properly understood. Elijah had to come to prepare the hearts of people so that they may be ready to receive the Christ when he came. To do this it was not necessary for Elijah to come in person, but that someone must come in the spirit and power of Elijah. (cp. Matthew 11:14; Mark 9:11-13 and Luke 1:17). Then Jesus continues by saying that Elijah had come. One had come in the spirit and power of Elijah. Then the disciples understood. Matthew reports that the disciples understood that Jesus was speaking of John the Baptist.
John fulfilled the prophecy of the return of Elijah because he came in the spirit and power of Elijah. He went before Jesus preparing the hearts of the people by preaching repentance, and pointing people to the Lamb of God who could answer the need of salvation that repentance revealed.
The fact is that Elijah comes again from time to time, and he will come again in this fashion, that is in the spirit and power of Elijah, because every time God grants a time spiritual refreshing and revival people need their hearts prepared by the message of repentance being preached and applied by the Holy Spirit. Unless people understand their lost condition due to their sin, and that they can't save themselves, they will be unable to appreciate Christ and his atoning grace. No doubt just before Christ comes again in glory to judge the living and the dead, one will be sent in the spirit and power of Elijah to prepare people for Christ's return, and so they may be ready, and not like the foolish virgins in Matthew 25 who were not ready and were shut out of heaven.
Jesus points out the tragedy of the his own time. Elijah had come in the person of John the Baptist, and they Jews in the main did not recognize him. Instead of receiving and welcoming Christ, they took him and killed him. This tragedy happens again and again. People do not recognize the coming of Elijah before times of special spiritual blessing, and so when God pours out his Spirit, they do not recognize the blessing of God, but rather oppose it.