There used to be false prophets among God's people, just as you will have some false teachers in your group. They will secretly teach things that are wrong -- teachings that will cause people to be lost. They will even refuse to accept the Master, Jesus, who bought their freedom. So they will bring quick ruin on themselves.
Many will follow their evil ways and say evil things about the way of truth.
Those false teachers only want your money, so they will use you by telling you lies. Their judgment spoken against them long ago is still coming, and their ruin is certain.