Matthew 17:1-13  Jesus takes Peter, James, and John up to a mountain .  They are given a vision that they were asked to keep to themselves until the resurrection.  The first thing that they see is Jesus transfigured. Jesus is changed before their eyes.  His face shines "like the sun".  His garments become white like light.  Later in his life, John, will again see Jesus in this form.  Revelation 1:13-17 describes Jesus as "one like a son of man" (human form), clothed in a robe that reaches His feet. "And His head and His hair were like white wool, like snow; and His eyes were like a flame of fire;"
He goes on to talk about Jesus' glowing feet, His majestic voice, and His face shining like the sun.
This is the glorified body of Christ - in His Kingdom.  ( Some believe that this is what Jesus was talking about in Matthew 16:28. These men saw the Son of Man coming in His Kingdom.)
The second thing that the 3 disciples see is heavenly visitors - Moses and Elijah talking with Jesus.
(I  have always wondered how they knew it was Moses and Elijah.  But they did. )  Peter seems to think that this is IT!  This is the coming of the Kingdom!  He wants to build tabernacles - tents for the three. But, before he can even finish his sentence, he is interrupted by the Father in Heaven.   A bright cloud, a fear inspiring voice... bring the disciples to fall on their faces.   "This is My beloved Son, with whom I am well-pleased; listen to Him."  This is the second time that the Heavenly Father has announced His pleasure in His Son.  The first time was at His baptism ( Matt. 3:17) and now, here at His transfiguration.

The disciples must have pondered the meaning of this vision as they walked down the mountain.  They ask Jesus about Elijah in particular.  The scribes taught that Elijah must come first before the Kingdom is restored and the Messiah reigns.  Jesus gives them an interesting answer.  Yes, Elijah is coming.. and yet, Elijah had already come - John the Baptist was the "Elijah" that was prophesied to come.  Many think that the two witnesses that are mentioned in Revelation 11 are Elijah and Moses.
It appears that Elijah has more work to do in the future of God's Kingdom.  Perhaps that is why he was taken up in a fiery chariot rather than just dying like everyone else? (II Kings 2:11)

I find myself also pondering this vision.  Jesus seems to be making it clear to his disciples that He is the Messiah,  that He is not just a man, and that He will be killed and raised from the dead.  He is preparing them for what is going to happen in just a short time.  We know they don't get it, that they won't get it until it actually happens, but what do we need to glean from this?   Jesus the Christ, the King of kings and Lord of lords will return to earth to reign.  Are we as hard of hearing as the disciples were back then?  Maybe if God spoke from a bright cloud we would get it?   They didn't.
But surely we would, wouldn't we?

Father in heaven.  Let Your Name be hallowed.  Let Your Kingdom come.  Let Your will be done, on earth as it is in Heaven.  We want to have ears to hear and eyes to see.  To be ready for the return of Your Beloved Son.  O to see Him in His glory!  To be transformed from this mortal and decaying body into one that is immortal and eternal.  To be forever with You in Heaven.   Thank You for  You loving kindness and grace.  God of peace,  will You, Yourself sanctify us entirely. Will You preserve us,  spirit, soul and body.. without blame at the coming of our Lord Jesus Christ.  Faithful are You who calls us and faithful are You to bring it to pass.  ( I Thess. 5:23-24) Amen!