I've been reading Stanley Hauerwas' commentary on Matthew as the lectionary has led us through this gospel. And I'm glad that I am.And a voice from heaven said, “This is my Son, the Beloved, with whom I am well pleased.”
For example, he points out that "six days later" points us to the creation story so that Jesus' transfiguration is the seventh day thus bringing God's work to completion.
Peter, James, and his brother John are there on the mountaintop with Jesus. They witness his transfiguration. They see him changed, and they see Moses and Elijah. Peter, as usual, speaks first. He offers to build three dwellings. We aren't told explicitly what Peter had in mind, but we can speculate that he wanted to keep Jesus on the mountaintop--usually a significant place in the Scriptures--along with those two important figures, Moses who had led the people out of slavery and brought them the law from the Lord and Isaiah the prophet who told them the Lord's word as they emerged from exile.
Perhaps Peter was attempting to live out this passage from Leviticus 23:
33The Lord spoke to Moses, saying: 34Speak to the people of Israel, saying: On the fifteenth day of this seventh month, and lasting seven days, there shall be the festival of booths to the Lord. 35The first day shall be a holy convocation; you shall not work at your occupations. 36Seven days you shall present the Lord’s offerings by fire; on the eighth day you shall observe a holy convocation and present the Lord’s offerings by fire; it is a solemn assembly; you shall not work at your occupations....
As Peter was still speaking, a bright cloud overshadowed them, and from the cloud, a voice speaks to them, repeating the words spoken at his baptism (Matthew 3:13-17) "This is my Son, the Beloved; with him I am well pleased" and adds some words not heard at that time, "listen to him!"
The immediate reaction of the disciples is fear. They fall to the ground.
They have seen light, heard the voice, and they are scared.
Jesus responds to their fear by coming to them and touching them. He tells them to get up and not to be afraid. Thomas Long, in his excellent commentary on Matthew, reminds us that Jesus has touched the leper (8:3), the hand of the fevered woman (8:15), and the eyes of the blind men (9:29) and healed them. Thus, as Jesus can heal blindness, fever, and leprosy, so can he heal fear.
When they look up, Moses and Isaiah are not visible. It's time to leave the mountain. On the way down, Jesus tells them not to tell anybody about the vision they have witnessed until after the resurrection.
Fred Craddock, in Preaching through the Christian Year A, explains:
If the disciples understood who Jesus was only after the resurrection there certainly was no reason to assume the crowds could. After all, if the baptism and prediction of passion seemed a contradiction of the terms "Messiah" and "Son of God," how much more would the cross? The people are not ready for the Transfiguration story because the disciples are not ready to tell it.