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 3:17-18
"Ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness. But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen"