Overall meaning : None but angels have We made the nineteen keepers of the Fire, nor have We specified their number but as a trial for the unbelievers and so that those to whom the Torah and the Gospel were given may be convinced that the Qur’an is the truth and that Muhammad is truly Allah’s Messenger, and so that the true believers may grow yet more firm in their faith by firmly believing such new piece of information, and so that those to whom the Torah and the Gospel were given and the true believers may not doubt what Allah has said regarding the number of the Fire keepers, and so that those in whose hearts is a disease of doubt and hypocrisy and the unbelievers who deny Allah and His Messenger may ask, ‘What could Allah mean by this number?’
Just as Allah has left the unbelievers to stray by mentioning this number and guided the believers to believe it, so too He leaves whomever He wills to stray and guides whomever He pleases. None knows the forces of your Lord and their huge numbers except Him. This is no more than an admonition to mankind so that they may take heed.