6 “And when they heard it, they fell on their face, and were sore afraid. And Jesus came and touched them, and said, Arise, and be not afraid. And when they lifted up their eyes, they saw no man, save Jesus only.”
How was it that, when they heard these words, they were dismayed? And yet before this also a like voice was uttered at Jordan, and a multitude was present, and no one felt anything of the kind; and afterwards again, when also they said, “It thundered,” yet neither at that time did they experience anything like this. How then did they fall down in the mount? Because there was solitude, and height, and great quietness, and a transfiguration full of awe, and a pure light, and a cloud stretched out; all which things put them in great alarm. And the amazement came thick on every side, and they fell down both in fear at once and in adoration.
But that the fear abiding so long might not drive out their recollection, presently He puts an end to their alarm, and is seen Himself alone, and commands them to tell no man this, until He is risen from the dead.
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.” what they were about.
Source: Homilies on the Gospel of St. Matthew (New Advent)