From any one who will teach them. See on 1Ti 5:13. It is a graphic picture of a large class, by no means extinct, who are caught and led by the instructions of itinerant religious quacks.
Never able (μηδέποτε δυνάμενα)
Because they have not the right motive, and because they apply to false teachers. Ellicott thinks that there is in δυνάμενα a hint of an unsuccessful endeavor, in better moments, to attain to the truth.