1 John 4

1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world.
If someone claims to come from God, test his/words teachings and lifestyles against scripture since this world loves encouraging what makes it happy.

5 They are from the world and therefore speak from the viewpoint of the world, and the world listens to them.
False teachers are popular with the world because just like false prophets of the Old Testament, they tell people what they want to hear. If a preacher or teaching becomes too popular, it might be wise to search for theological errors.