This is one of those Scriptures that is sneaky important. If you look at vss.19-21 you will see that this Scripture is one of those that people use to stress the authority of Scripture. I am one of those people. I do believe that the Bible is much more than just a history book. I do believe that the Bible was written with much more authority than any other book which we have. I do believe that the Bible is inspired by God and contains inerrant truths. When you look at vs.20 most of us interpret it as stating that we are not free to come up with an interpretation of Scripture that does not reflect the character of God nor the historical witness of God’s Word. That is crucially important.
If you go back to vs.16 and you read that “we did not follow cleverly devised myths when we made known to you the power and the coming of our Lord Jesus Christ”, he is speaking about being an eye witness to what Jesus was able to do. Remember, this is II Peter, so it is the second letter written by Peter, the disciple that many call primary. We read in vs.17 of the account that Peter had with James and John when he climbed the mountain and Jesus was transfigured. For the transfiguration Scriptures you can read them at: Matthew 17:1–8, Mark 9:2–8, Luke 9:28–36 .
Peter is speaking from the viewpoint of an eyewitness who not only believes that Jesus is the Messiah, but he also experienced the living, dead, and resurrected Jesus. One of my favorite scenes of Jesus and Peter is found when Peter is rehabilitated in John 21:15-19 where Jesus takes Peter and his betrayal and turns it around into a beautiful time of reconciliation and healing. That is the perspective from which it is written.