For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist. Watch yourselves, so that you may not lose what we have worked for, but may win a full reward. Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works.
What's the most harmful evil for followers? Deceiving spirits is a term I just learned. It means that those demonic spirits, either directly or through false teachers, who have wandered away from the truth and lead others to do the same. It will be tremendously pathetic that professing or nominal believers who associate with those who truly believe the gospel but defect after believing lies and deception. The reason is that all their efforts of faith pursing might not convert their sinful nature into integrity. Moreover, they will have to take upon others' wicked works at the same time. Thus, it's important to keep an eye on discerning the doctrines of demons that are not about teaching demon's materials, but false teaching that originate from them. False teaching always covers up the facts, but true teaching reveals the truth. Sometime it might be not easy to discriminate between them. At least for all I know, true teaching is no way defeated by interrogation. In contrary, a man with responsible attitude should always query absurd things before they make conclusion. Like an old saying is that "True gold fears no fire, true blue will never stain".