18. (Verses 12-13 )clarify two things about the deities whom the mushriks invoke. Firstly, they can do a person neither any good nor any harm. Nay, it is more probable that they do harm rather than good. For when the mushrik invokes other deities than Allah, he loses his faith forthwith. Secondly, the mushrik himself knows that there is no guarantee or probability of any good from his god who is utterly helpless and powerless. As regards to the occasional grant of his request through his god, this is done by Allah merely to test his faith.
19. That is, the one, who leads a person to the way of shirk is the worst guardian and the worst comrade, whether he be a human being or a satan.