There were false prophets in Israel, and Peter promised that false teachers would exist in the church. What is the difference between a false prophet and a false teacher? In reality, nothing at all. A prophet is one inspired of God. A false prophet may claim to be inspired, but his message is his own. A preacher of truth is not inspired, but speaks in accord with inspiration. A false teacher claims to preach truth, but his message is his own. Such men may evidence characteristics that are worldly and evil, but they are known by their fruit (their doctrine, cf. Matthew 7:15-20), which does not accord with righteousness.
Peter warns about such men because some would follow their teaching, contradicting truth. They would exploit Christians, convincing some that their message was from God when it was not.
It is obvious that such men will come into judgment. However, their doctrines destroy those who follow them as well. This is why Peter warns of them, and why we must as well.
