All of the apostles were eyewitnesses and earwitnesses of Jesus Christ but only Peter, James, and John were with Jesus when he was transfigured on the mountain (Mark 9:2). The transfiguration is the majesty that Peter is referring to (as will be confirmed in 2 Peter 1:17–18). Details of the transfiguration can be found in Matthew 17:1–5, Mark 9:2–7, and Luke 9:28–35.
16 For we did not follow cleverly devised myths when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty.