Now six days later, Jesus took Peter, James, and John, and led them up on a high mountain to pray; and He was transfigured before them.
He took Peter, James, and John as eyewitnesses; to observe and record the transfiguration.
So as he was praying, the appearance of his face changed, and his clothes became dazzling white, and suddenly, two men appeared, Moses and Elijah were talking with him. Luke 9:31 says, "They appeared in glory and were speaking of his departure, which he was about to accomplish in Jerusalem."
And a cloud came and overshadowed them; and a voice came out of the cloud, saying, "This is My beloved Son. Hear Him!"
Suddenly the disciples, looking around, they no longer saw anyone with them except Jesus. As they were coming down the mountain, he ordered them to tell no one what they had seen until the Son of Man had risen from the dead. So they kept this word to themselves, questioning what the rising from the dead meant.
Later on after Jesus had risen from the dead Apostle Peter wrote, "we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. And this voice which came from heaven we heard, when we were with him in the holy mount." 2 Peter 1:16-18.
Now, the transfiguration happened to confirm Peter’s great confession, "Thou art the Christ" Mark 8:29.
Earlier he had asked his disciples, "Whom do men say that I am?" "And they answered, John the Baptist: but some say, Elias; and others, One of the prophets." Mark 8:28. "And he saith unto them, But whom say ye that I am? And Peter answereth and saith unto him, Thou art the Christ."
Now the transfiguration happened to clear the air, and Elijah and Moses appeared representing the Prophets and the Law, talking with Jesus of how he would be glorified, and a voice coming out of the cloud, saying, "This is My beloved Son. Hear Him!"
With all that said and done, Peter, James, and John, saw or witnessed the Kingdom of God arrive in great power at the mount of transfiguration.
Simple Gospel Summation