And We have set none but angels as Guardians of the Fire; and We have fixed their number only as a trial for Unbelievers,- in order that the People of the Book may arrive at certainty, and the Believers may increase in Faith,- and that no doubts may be left for the People of the Book and the Believers, and that those in whose hearts is a disease and the Unbelievers may say, "What symbol doth Allah intend by this?" Thus doth Allah leave to stray whom He pleaseth, and guide whom He pleaseth: and none can know the forces of thy Lord, except He and this is no other than a warning to mankind.