"Now about eight days after these sayings Jesus took with him Peter and John and James, and went up on the mountain to pray. And while he was praying, the appearance of his face changed, and his clothes became dazzling white. Suddenly they saw two men, Moses and Elijah, talking to him. They appeared in glory and were speaking of his departure, which he was about to accomplish at Jerusalem. Now Peter and his companions were weighed down with sleep; but since they had stayed awake, they saw his glory and the two men who stood with him. Just as they were leaving him, Peter said to Jesus, ‘Master, it is good for us to be here; let us make three dwellings, one for you, one for Moses, and one for Elijah’—not knowing what he said. While he was saying this, a cloud came and overshadowed them; and they were terrified as they entered the cloud. Then from the cloud came a voice that said, ‘This is my Son, my Chosen; listen to him!’ When the voice had spoken, Jesus was found alone. And they kept silent and in those days told no one any of the things they had seen."We are left to speculate as to why Peter, John, and James kept silent. They were not instructed by the Lord to do so. I always thought that they were afraid that if they spoke, no one would believe them, or worse, they would be accused of blasphemy. After the resurrection and the reception of the Holy Spirit, these three were transformed and given the courage to testify as to what had occurred on that mountain.
Matthew Henry (1662 – 1714) in his Commentary puts it this way,
"Lastly, The apostles are here said to have kept this vision private. They told no man in those days, reserving the discovery of it for another opportunity, when the evidences of Christ’s being the Son of God were completed in the pouring out of the Spirit, and that doctrine was to be published to all the world. As there is a time to speak, so there is a time to keep silence. Every thing is beautiful and useful in its season.""Everything is beautiful"? Sorry but I couldn't resist,
I cannot keep silent. I have seen a lot of ugliness and can testify that not everything is beautiful.