The Repentance - 9:16
Do you think that you will be left until Allah knows and distinguishes between those among you who strive hard for Him and those who take none as close friends except Allah and His Messenger and the believers? Allah is All-Knowing of what you do.
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.