The Repentance - 9:71
The believing men and the believing women are friends of one another. They enjoin good, forbid evil, observe the prayer, pay the zakat, and follow Allah and His messenger. Allah will have mercy on them. Allah is Superior, Wise.
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.