Peter was there and saw it himself, along with two others:
Therefore I will not be negligent to remind you of these things, though you know them, and are established in the present truth. I think it right, as long as I am in this tent, to stir you up by reminding you; knowing that the putting off of my tent comes swiftly, even as our Lord Jesus Christ made clear to me. Yes, I will make every effort that you may always be able to remember these things even after my departure. For we did not follow cunningly devised fables, when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. For He received from God the Father honor and glory, when the voice came to Him from the Majestic Glory, “This is my beloved Son, in whom I am well pleased.” This voice we heard come out of heaven when we were with Him in the holy mountain.
–2 Peter 1:12-18
What did they see? Here is Matthew’s account of what they saw:
After six days, Jesus took with him Peter, James, and John his brother, and brought them up into a high mountain by themselves. He was transfigured before them. His face shone like the sun, and His garments became as white as the light. Behold, Moses and Elijah appeared to them talking with Him. Peter answered, and said to Jesus, “Lord, it is good for us to be here. If you want, let’s make three tents here: one for you, one for Moses, and one for Elijah.” While he was still speaking, behold, a bright cloud overshadowed them. Behold, a voice came out of the cloud, saying, “This is my beloved Son, in whom I am well pleased. Listen to Him.” When the disciples heard it, they fell on their faces, and were very afraid. Jesus came and touched them and said, “Get up, and don’t be afraid.”
This fisherman, Peter, saw the majesty of God’s Anointed Son…Jesus. Years later, as Peter neared death, he could be found continuing to describe to people what he saw and heard back on that mountain. What he had witnessed had changed him and given birth within him a conviction that would remain until, and through the midst of, his murderous death by the Romans. After seeing the Son of God, Peter had been ruined to his old self and to the careless world around him. Peter saw the revelation of the glory, honor and majesty of the Son of God, and he needed to tell everyone who would listen before death would silence him. From that day on, Peter “remembered the Lord.” It was not just what he did, but it was what he was from the changed depths of his being. The burning conviction within him needed to make sure that, after his death, we could read and understand his eye witness account and, with the convicting power of the Holy Spirit, come to believe into and experience Jesus for ourselves. The knowledge of the Son of God can be ours — first hand and experiential — if we will humble ourselves to receive the eye witness testimony and hear what the Son of God is saying… if anyone thirsts!