We see in the epistles of the New Testament rather frequent warnings against false teachers. Of course, these characters were not new to the New Testament. Jesus said in Matthew 7:15, "Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves." He gives instructions on how to identify these enemies in verses 16-19 and sums it up with verse 20, "Wherefore by their fruits ye shall know them."

Then Jesus issues some sobering words about these false teachers who may appear to be speaking in the name of Jesus, but are actually unregenerate:

Matthew 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
Matthew 7:22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
Matthew 7:23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.

The Apostle Paul spent much of his writings trying to counteract the harm that the false teachers were doing, particularly the gnostics and judaizers of his day. Peter, John and Jude were all very definite in their warnings against the dangers of false teachers of their day.