The Cow - 2:257
Allah is the sovereign and the friend of those who believe; He brings them out of darkness into light. But the friends of those who disbelieve are fierce men; they bring them out of light into darkness. They are the people of the Fire, they will abide therein forever.
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.