We have specified their number only as a trial for those who are bent on denying the truth , so that those who were given the Book might gain in certainty , and those who believe might increase in faith -- and so that neither those who have been given the Scripture nor the believers might have any doubts , and that those sick at heart and those who deny the truth might ask , " What does God mean by this parable ? " In this way , God lets go astray whom He wills , and guides whom He wills .