Two or three years after he wrote his first letter, Peter had heard worrying news that false doctrine was being taught in the churches of Asia Minor (modern day Turkey). These people had already caused problems, and Peter feared that their sinful ways would continue to sully the Christian churches. Therefore, he wrote this letter from his prison cell in Rome, to warn against these false teachers. Christians should know their Bible, their enemy, and what lies in store for us all in the future.
II Peter 1:16-18
"For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. And this voice which came from heaven we heard, when we were with him in the holy mount"