“I am surprised at you! In no time at all you are deserting the one who called you by the grace of Christ, and are accepting another gospel”. 

Gal 1:6 GNB 

A problem arises for many teachers when people over whom they have expended time and effort and whom they have encouraged into personal growth then turn round and deny the things they had learnt. It comes as a “slap in the face”. It seems like so much wasted effort. In addition it is experienced as disloyalty and often there is personal hurt. 
That is what happened in the Galatian churches Paul had founded. He had expended a lot of physical effort to get there in the first place – by walking. Then he had, in love, offered Christ to the people there. They had accepted Christ and given their lives to him in faith and entered the Christian way of life. When Paul left to bring the gospel of Jesus Christ to other people the trouble began. Other teachers came in and undid the good Paul had done. And instead of remaining loyal to him, many people swallowed the beliefs these other teachers were talking about. They were a contradiction of the basic elements of the Christian gospel that Paul had been at such pains to share with them. He was devastated. 
False teaching is always a danger in the Christian community. It is being spread by books, separated groups with adulterated doctrine, and people who often have no idea what they are talking about. Frequently it is sheer jealousy that motivates them. Sometimes it is money. Get to know the right beliefs and doctrine of your Christian fellowship and test any new ideas by reference to the pure gospel of Jesus. 

Lord, keep the truth of the gospel pure in our day.