Matthew 17:9

He enjoins silence respecting what they had seen, for this reason, that when they should be filled with the Holy Spirit, they should then become witnesses of these spiritual deeds.

For "as they came down from the mount, He charged them to tell the vision to no man, until He were risen from the dead." For the greater the things spoken of Him, the harder to be received by the generality at that time; and the offense also from the cross was the more increased thereby. Therefore He bids them hold their peace; and not merely so, but He again reminds them of the passion, and all but tells them also the cause, for which indeed He requires them to keep silence. For He did not, you see, command them never to tell any man, but "until He were risen from the dead." And saying nothing of the painful part, He expresses the good only. What then? Would they not afterwards be offended? By no means. For the point required was the time before the crucifixion. Since afterwards they both had the spirit vouchsafed them, and the voice that proceeded from the miracles pleading with them, and whatsoever they said was thenceforth easy to be received, the course of events proclaiming His might more clearly than a trumpet, and no offense of that sort interrupting what they were about.

And as they came down from the mount, Jesus charged them, saying, Tell no man this vision, until the Son of Man shall rise from the dead. He will not be preached among the people, lest the marvel of the thing should seem incredible, and lest the cross following after so great glory should cause offence.

Or, because if His majesty should be published among the people, they should hinder the dispensation of His passion, by resistance to the chief Priests; and thus the redemption of the human race should suffer impediment.