There were also false prophets among the people

2 Peter 2:1-9
But there were also false prophets among the people, just as there will be false teachers among you. They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them—bringing swift destruction on themselves. Many will follow their depraved conduct and will bring the way of truth into disrepute. In their greed, these teachers will exploit you with fabricated stories. Their condemnation has long been hanging over them, and their destruction has not been sleeping.
For if God did not ... if this is so, then the Lord knows how to rescue the godly from trials and to hold the unrighteous for punishment on the day of judgment. 
The argument of this chapter is closely paralleled in Jude 4-16. To be forewarned his readers against false teachers who have already disrupted other Christian groups. 
Why?
False teachers were warning by Peter, John, and Paul. So this is an existing problem in the first century.
The false teacher is caused by sins, greed and human's proud heart.
God won't allow false prophets and false teachers among us, the condemnation has been hanging over them, and their destruction has not been sleeping. Jesus as God saves Noah his family, 8 people.
How?
Be aware of the false teacher and follow God's word with Holy Spirit.