Matthew 17:9

And as they were coming down from the mountain, Jesus commanded them, saying, Tell the vision to no man, until the Son of man be risen from the dead.

Silence was commanded because nine of the apostles had not witnessed the transfiguration, and there was a possibility of jealousy developing among them, as indeed it did a little later; also the primary reason, as noted earlier, was the need not to compromise the Saviour's death which he would soon accomplish in Jerusalem.

An extremely important supplement to Matthew's account is in Luke who gave the subject matter of the conversation between Jesus and Moses and Elijah. "(They) spake of his decease which he was about to accomplish in Jerusalem" (Luke 9:31). This conversation was calculated to encourage and reassure the apostles who had been severely shocked and disheartened by the Saviour's revelation of his death and sufferings, to be followed by his resurrection. It seems that the apostles focused all their attention upon his death and continue not to realize, though they had been told, that he would also rise from the dead. The subject matter during the transfiguration showed that the death of Christ was a part of the Master Plan and that it was of the utmost concern and interest on the part of all previous generations as represented by Moses and Elijah. It also revealed Christ as the Great Architect of the crucifixion. Evil men, dominated by Satan, would have their part in it, but only Christ would accomplish his death. These events, coupled with the sublime conversation, should have enabled the apostles more readily to accept the somber events of his approaching passion.