Its in the verse you cut off. Let me highlight it for you.
Mat 17:3 And, behold, there appeared unto them Moses and Elias talking with him.
Mat 17:4 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt,let us make here three tabernacles; one for thee, and one for Moses, and one for Elias.
Mat 17:5 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
Mat 17:6 And when the disciples heard it, they fell on their face, and were sore afraid.
Mat 17:7 And Jesus came and touched them, and said, Arise, and be not afraid.
Mat 17:8 And when they had lifted up their eyes, they saw no man, save Jesus only.
Mat 17:9 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.
Shows they appeared to them. Moses, Elijah, with Jesus speaking of his decease (in Luke 9:31) which he was to accomplished in Jerusalem, which is also what the law and the prophets had spoken of ( Luke 24:26-27, Acts 3:18).
And upon the mount the Father spoke to them out of the cloud just as Peter expressed the desire to build tabernacles after the pattern he was seeing on the mount, and perhaps thinking along the lines of Moses when he was upon the mount aforetime instructed to do so ( Ex 25:40, Heb 8:5). But here it would be for what the Lord pitched and not man Heb 8:2 even as Moses who wrote of Christ (John 5:46) was also a servant and testimony of the things that would be spoken of after (Heb 3:5). As God speaking by Moses testified of Christ and hearing him ( Duet 18:18) as He speaks the same in Mat 17:5 to hear his son. After the voice was passed they looked up and the two men who were with him (Moses and Elijah) vanish. The Law which come by Moses and Elijah as the law and the prophets prophesied until John who was Elijah Mat 11:13 for to come Mat 11:14. These two men (acccording to the vision) vanished after the Father had spoken (to hear his beloeved Son) and leaving Jesus alone ( even, the righteousness of God) without the law having been manifest and he himself being witnessed by the law and the prophets in the same. And they were charged to tell no man until Son of man was raised from the dead (Mat 17:9) when it is openly declared that he is the Son of God with power, according to the spirit of holiness, by the resurrection from the dead (Romans 1:4) and in that way did they speak of it as it was permitted them to do so at the time as they did so.
Mat 17:9 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.
And after he was raised from the dead (Romans 1:4) they say, "BUT NOW"
Romans 3:21 But now the righteousness of God without the law is manifested,
being witnessed by the law and the prophets;
Which would seem to be a part of fulfilling, what is called "the vision of all" concerning Christ even,
He who cometh in the volume of the book
Isaiah 29:11 And the vision of all is become unto you as the words of a book that is sealed, which men deliver to one that is learned, saying, Read this, I pray thee: and he saith, I cannot; for it is sealed:
The effect of every vision, the vision they saw on the mount was in accord with the vision of all (the law and the prophets)
