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 1:13-14
"Yea, I think it meet, as long as I am in this tabernacle, to stir you up by putting you in remembrance; knowing that shortly I must put off this my tabernacle, even as our Lord Jesus Christ hath shewed me"