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 3:11-13
"Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness, looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat? Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness"