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 1:1-3
"Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ: Grace and peace be multiplied unto you through the knowledge of God, and of Jesus our Lord, according as his divine power hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue"