Peter wrote that the gospel message that he and the other apostles preached was not man made myth, but eyewitness testimony. He knew that he would not live long in the flesh, so he was determined to make this clear before his passing. He saw the unveiled Jesus on the Mount of Transfiguration, he witnessed the empty tomb and the risen Savior, he joined him walking on the water. Peter knew the message he shared was true because he was there to see it. We can know that it is true by believing his testimony.