Jesus had foretold this event when he said six days earlier that some of them would not taste death until they saw the Son of Man coming in his kingdom
(Mat 16:28–17:1, Mark 9:1–2a, Luk 9:27–28).
This is fulfilled through the transfiguration of Jesus which would be a small foretaste of the coming glory of God's kingdom:
Jesus and three of his disciples; Peter, John, and James went to a mountain
(Mount of Transfiguration) to pray. On the mountain Jesus began to shine with bright rays of light. The prophets Moses and Elijah then appeared beside him and Jesus spoke to them. They spoke of Jesus' decease, which he was about to accomplish in Jerusalem (Luk 9:30–31). A bright cloud covered them, and a voice from the cloud said (a voice from God the Father): "This is my beloved Son, my Chosen One; with whom I am well pleased;
listen to him." (Mat 17:2–8, Mark 9:2b–8, Luk 9:29–36a, John 1:14b).
This is a similar statement to what God the Father said at the baptism of Jesus, see John the Baptist in The Prophecies of Isaiah. There are similar statements in the Old Testament (Psalm 2:7, Mat 12:18, Isaiah 42:1, Deu (5 Mos) 18:15).
Jesus commanded them not to tell anyone this vision (which was probably more than a vision), until he rose from the dead (Mat 17:9, Mark 9:9, Luk 9:36b).
When king Solomon built the temple, it was filled with a cloud and the priests couldn't perform their service because of it, for the glory of the Lord filled the temple of God (2 Chron 5:13–14, 7:1–3). It was the presence of the Lord that then came into the temple. In a similar way the presence of the Lord filled the area with these three disciples.
Jesus said at another occasion that true worshipers should worship
God the Father in spirit and truth, for the Father seeks such people to worship him. God is spirit, and those who worship him must worship in spirit and truth
(John 4:23–24). See also How should we worship God? in
The Woman's Place in the Congregation.
Peter, John, and James saw the glory that Jesus set aside to become a man. They were eyewitnesses of his majesty (2 Pet 1:16–18). It's probably the appearance that Jesus will have in the millennial kingdom and beyond.
The disciples needed a demonstration that showed who Jesus really is,
God and man in one person, see Jesus is God.
Moses is the law giver of the Mosaic Laws for the Jewish people. Technically it's the law of God, but because it is so closely connected to Moses it's called the
Mosaic Laws. Elijah represents the prophets of the Old Testament. He can be seen as the prophet of prophets. He is also a prophetic sign of the last days
(Malachi 4:1, 5–6), see The Book of Malachi and The Day of the Lord.
These two men represent what the Bible calls "the Law and the Prophets", and are what the Jews call the ancient Hebrew Scriptures, which are the main part of the Old Testament.
Jesus had told the disciples that some of them would experience the kingdom of God (Mat 16:28, Mark 9:1, Luk 9:27). Peter, John, and James got a preview of this kingdom. As the kingdom of God is manifested, knowledge increases
(1 Cor 13:12), see the end part of Spiritual Gifts. These three disciples
therefore immediately understood that these persons were Moses and Elijah
without receiving any introduction.
Moses and Elijah hadn't yet received their new bodies that they will receive at
The Rapture. They had no physical bodies. Moses and Elijah were in Paradise, see The Rich Man and Lazarus. Despite their absence of bodies these disciples
immediately understood who they were, persons they hadn't seen before.
There was a knowledge that surpasses anything we have today through intellectual learning or observations through our five senses (Isaiah 11:9).
The kingdom of God is within us (Luk 17:20–21 (NKJV)). It starts when you become a Christian believer, see Salvation. There will be a time when God's kingdom is manifested on the new Earth, see A New Heaven and a New Earth.
Before that it will be seen when Jesus returns, see
The Millennial Kingdom and the Great White Throne Judgement.
God the Father said on the mountain that they should listen to Jesus
(Heb 1:1–2, Acts 10:43), see Jesus and the Law. But the disciples still had a hard time understanding why Messiah had to die on a crosss (Mark 9:9–10).
It was hidden from them, so that they wouldn't perceive it (Luk 9:43–45).
They saw him only as a deliverer, not as the suffering Messiah. This changed after The Resurrection of Jesus, see The Great Commission.
See also Some stories in the gospels at the bottom of The Four Gospels.
