By the teaching you will know the teacher. The one who teaches Jesus Christ crucified and risen for the sin of the world, who points you to Christ and His Word for eternal life is a true teacher and shepherd. He is a good tree that bears good fruit. The one who teaches you to rely on your own works of the Law, your decisions, your feelings, is a false teacher and a wolf in sheep's clothing, a bad tree bearing bad fruit. To build a life on such teaching is to build your house on shifting sand.
Bad doctrine has a way of shifting around like loose grains of sand blown about by the wind.
First it teaches one thing, then it teaches something else. New ideas bring on new teachings and new teachers and new books and videos. People flock to it, until something even newer comes along. You hear that said in places where there is no formal doctrine, where the teachings of Scriptures are never summarized and confessed publicly. Truth that is not true for all people at all times is no truth at all, but opinion.