The Iron - 57:16
Is it not time for the believers to open their hearts to the Message of Allah and the truth that has come down? Let them not be like those to whom the Book was given before; their hearts were hardened when a long time had passed over them. The majority of them are those who have gone 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.