Peter, James, and John were helped in their understanding of who Jesus was by the events on the mountaintop. They witnessed a change in Jesus' body. They saw him clothed in white as was the Divine One described by Daniel (7:9). And they saw Moses and Elijah, great figures in the history of their people, both who had spoken the word of the Lord.
The first reading this week, 2 Kings 2:1-12 depicts the departure of Elijah. John H. Hayes, in Preaching through the Christian Year B, suggests reading Exodus 24 and 34 to see how Mark's version of the Transfiguration is patterned on Moses' experiences of God on Mt. Sinai.
Then a cloud overshadowed them. And from the cloud came a voice.
What Jesus has known (1:11) is now told to these disciples: The voice tells them, "This is my Son, the Beloved; listen to him!"
And what he first tells them is not to tell anybody else until after the Son of Man had risen from the dead. Fred Craddock in Preaching through the Christian Year B, explains this command: They may have heard that Jesus is the Son, the Beloved, but they haven't yet heard everything they need to know to be effective disciples. They are not yet ready to be witnesses nor are their audiences yet read to hear it.
Off on a tangent: Look back at Exodus 19:16-20 that tells of the appearance of a thick cloud on a mountain and the voice of the Lord; also Exodus 24:12-18, Moses and Aaron go up the mountain, the glory of the Lord appears in the cloud, then Moses enters the cloud. Other references to the cloud as a symbol of the divine presence include Numbers 14:10; Ezekiel 1:4; Daniel 7:13-14; Mark 13:26; 14:2. (Thank you once more, Allen & Williamson, for your Preaching the Gospels.)
Sing (or, at least read) these two hymns about the Transfiguration: 258, "O Wondrous Sight! O Vision Fair" and 260, "Christ, upon the Mountain Peak," The United Methodist Hymnal.
All the elements of Mark's account are there: the six days of waiting, the cloud, the glory, the voice, the descent from the mountain. Moses' face shone due to his experience in the presence of God. Exodus also describes the making of the tent of meeting.
2 Corinthians 4:3-6
The three disciples have accompanied Jesus to the mountaintop. There, they witnessed something that terrified them.
The gospel reports of the transfiguration tell us that the disciples saw something very different from what they had been able to see before. I've always understood that to mean that the appearance of Jesus changed. As Mark says, "He was transfigured...and his clothes became dazzling white."
But, what about us? We've heard about Jesus. We know about the crucifixion and we know about the resurrection. Is transfiguration interesting only as an historical event that happened to some other people at some other time in some other place?
Paul was writing to the early church, the post-transfiguration, post-crucifixion, post-resurrection church: Those other apostles have been misleading you. They have thrown a veil over the true gospel. The light that has enabled us to see is the light that will enable you to see. That light comes in the glory of Christ, who is the image of God.
The light was there for the Corinthians, but they had allowed themselves to be blinded by the false apostles.