Prophet Jonah - 10:104
"Say: "O people, if you have any doubt about my religion, know that I do not serve those whom you serve besides Allah. I serve only Allah, Who took your life. I am commanded to be of the believers."
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.