“And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.”
King James Version (KJV)
17:9 Tell the vision to no man - Not to the rest of the disciples, lest they should be grieved and discouraged because they were not admitted to the sight: nor to any other persons, lest it should enrage some the more, and his approaching sufferings shall make others disbelieve it; till the Son of man be risen again - Till the resurrection should make it credible, and confirm their testimony about it.
Mt 17:9 Jesus charged them, saying, Tell the vision to no man. Even they themselves did not yet understand what they had seen. Still less could they, in present circumstances, make others understand. All was plainer after Christ had died, risen, and had ascended to glory. The time had not come to proclaim the mystery of the Sonship of the world.