Peter responded to some zany accusation from false teachers in his time. "False teachers were claiming that Jesus' resurrection and return, as well as the Holy Spirit's indwelling of believers, were all cunningly devised fables. Peter countered their faith claims with an eyewitness account." (from my NKJV Study Bible commentary) Peter was referring to Christ's transfiguration when he, James, and John were present with Christ on the sacred mountain. "In Christ's transfiguration the disciples received a foretaste of what His coming will be like when He returns to establish His eternal kingdom." (from my NIV Study Bible commentary) They heard the voice of God the Father saying, "This is My Son, whom I love; with Him I am well pleased." God the Father gave Jesus honor (the public acknowledgment of His Sonship) and glory (His face and clothes shone with glorious splendor). Peter had seen Christ's majesty and power manifested in His miracles with his own eyes and heard His teachings with his own ears; he knew that what he told about Jesus was true. This is to confirm the authenticity and reliability of Peter's gospel testimony/message.
This passage made me think of the song, He's Alive by Don Francisco. It's written from Peter's perspective.