The One Wrapping Himself Up - 74:31
"We have appointed only angels as guardians of the Fire, and We have made their number (nineteen) a trial for the disbelievers, to convince those who have been given the Book, to strengthen the faith of those who believe, to remove the doubts of those who have been given the Book and those who believe, and for those in whose hearts is a disease and the disbelievers to say, "What does Allah mean by this example?"So Allah leads astray whom He wills and guides whom He wills. None knows the armies of your Lord except Himself. This (number) is a message to the people."
The most important criterion of belief:
The Cattle/6:82
Those who believe and do not mix their faith with wrongdoing, they deserve security and they have found the truth.
Luqman/31:13
"When Lokman advised his son, he said: "My dear son, do not associate partners with Allah; surely shirk (associating partners with God) is a great wrong."
The Companies/39:65
It has been revealed to you and those before you that if you associate partners, all that you do will be in vain and you will be of the losers.