The Women - 4:135
Believers! Bear witness for the sake of Allah and observe justice, even against yourselves and your parents and your relatives. Whether rich or poor, Allah takes care of both groups. So do not be biased in accordance with your personal interests and feelings. If you distort the truth, or if you refrain from testifying, know that Allah is aware 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.