The Prohibition - 66:8
"O you who believe, repent sincerely to Allah so that your Lord may cover your evil deeds and admit you into Gardens beneath which rivers flow. Allah will not disgrace the Messenger and those who are with him on that Day. Their light will spread from their front to their right side and they will say: "Our Lord, complete our light for us and forgive us; You are the Almighty."
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.