While he was with Jesus throughout His ministry, Peter witnessed many events that “made more certain” the Good News of Jesus. He did not come to the confidence He had in Jesus just because He was at the Transfiguration. There was so much more Peter saw and heard:

• The blind receive sight, the lame walk, those who have leprosy are cured, the deaf hear, the dead are raised, and the good news is preached to the poor (Luke 7:22).
• In Acts 2:32 he said God has raised this Jesus to life, and we are all witnesses of the fact.
• He expressed his confidence this way a few chapters later: Salvation is found in no one else, for there is no other name under heaven given to men by which we must be saved (Acts 4:12).
• And Peter wrote … you know that it was not with perishable things such as silver or gold that you were redeemed from the empty way of life handed down to you from your forefathers, but with the precious blood of Christ, a lamb without blemish or defect. (1 Peter 1:18-19)

Peter understood Jesus to be the Son of God and the Savior. The Transfiguration was one part of that understanding. Read again his recollection of that event.

2 Peter 1:16-21 16 We did not follow cleverly invented stories when we told you about the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. 17 For he received honor and glory from God the Father when the voice came to him from the Majestic Glory, saying, “This is my Son, whom I love; with him I am well pleased.” 18 We ourselves heard this voice that came from heaven when we were with him on the sacred mountain.  19 And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts.  20 Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation.  21 For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.

The prophecy Peter mentions, what we call Scripture, is all about this message: God promised to save fallen mankind from their sins and carried out that promise in the person of His Son, who lived a life without sin and offered it on the cross in payment for your sins. We have the writings of the Old and New Testaments that have preserved that message and passed along the word made more certain for us today. We have the account of those who were eyewitnesses passed along to us so that we could know Jesus is the Son of God, as He revealed on that mountain of the Transfiguration.

Peter seemed to anticipate those of us living today asking, “How can I be sure about Jesus? I wasn’t there!” Peter is telling us, as only an eyewitness can, “I know you weren’t there, but I was. I saw Jesus shine like the sun. I had already seen Him perform miracles and heard Him claim to be the promised one. And then I heard the voice of the Father proclaim Jesus to be His Son, with whom He was well-pleased. It convinced me that He was the one the prophecies had pointed to!” And you don’t have to just take Peter’s word for it. Jesus Himself had said, You diligently study the Scriptures because you think that by them you possess eternal life. These are the Scriptures that testify about me (John 5:39). You will find eternal life as you search the Scriptures because they all point to Jesus, who is our salvation. The Word made more certain.

Notice that Peter did not say the prophetic word was made more true. It was already true, being God’s Word. However, it was made more certain, since Peter saw for Himself the fulfillment of the age-old promises. That is why he gives us this admonition: we have the word of the prophets made more certain, and you will do well to pay attention to it. You have all the assurances that you need in God’s Word and His Word made flesh, Jesus. So pay attention to it!