SO WHAT DID PETER SAY?
On that mountain, Peter just had to say something to the Lord. He did! Luke points out that he spoke, "Not knowing what he said" (Luke 9'33).
So reacting to what he saw he blurted out the first thing that came to him:
“If you wish, let us make here three tabernacles: one for You, one for Moses, and one for Elijah”.
But if Peter didn't know the implications of what he what was saying, the Father did:
"While he was still speaking, behold, a bright cloud overshadowed them;
and suddenly a voice came out of the cloud, saying,
'This is My beloved Son, in whom I am well pleased. Hear Him!'"
The Father made Himself very clear!
"And when the disciples heard it, they fell on their faces and were greatly afraid," Matthew 17'5-6.
Peter was suggesting that these Prophets, great though they were, would be on the same level as Jesus, God the Son, which was completely against God's will.
It would mean that their words would carry the same authority as His. So the Father interrupted to command them, and all future Believers, to “Hear Him!”.
Of course, the word of Christ is above Moses and Elijah or Jeremiah or Malachi especially regarding life and godliness (more on that later).
Had he/they forgotten?
The Disciples, and more particularly Peter, had heard Jesus place His word above all others before.
He had said: “I say to you” nine times in Matthew 5 alone (18, 20, 22, 26, 28, 32, 34, 39, 44).
There are His phrases too, like "he who hears these sayings of mine and does them”.
Also these words: "Heaven and earth will pass away but My words will by no means pass away,” Luke 21'33.
And, "No one comes to the Father except through Me,” John 14’6.
So when the Lord said, “but I say to you”, He meant it. Also, He asserted that His true followers would hear Him: “My sheep hear my voice" (John 10’27).
Reading, listening and doing the word of Christ is how a Believer comes to know the Father's will.