Acts 20:29-31

29I know that trouble will come to you after I leave. Bad people will come to teach you. They will cause trouble for the believers, like dangerous wild animals that attack sheep.

30Even men from your own group of believers will teach things that are not true. They want to deceive other believers, so that they leave you and join with them.

31So be very careful! For three years I taught you, in the day and in the night. I warned you to be careful, so that I even cried about it.