We have appointed only angels to be wardens of the Fire , and caused their number to be a stumbling block for those who disbelieve ; so that those given the Scripture may attain certainty ; and those who believe may increase in faith ; and those given the Scripture and the believers may not doubt ; and those in whose hearts is sickness and the unbelievers may say , “ What did God intend by this parable ? ” Thus God leads astray whom He wills , and guides whom He wills .