The Iron - 57:27
Then We sent Our messengers after them, one after another. We also sent Jesus the son of Mary and gave him the Gospel, and We put compassion and mercy in the hearts of those who followed him. But they invented for themselves a spirituality which We did not approve of them. Whereas We had asked them to follow only that which would please Allah. And they did not observe it properly. We gave the believers among them their reward, but many of them were astray.
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.