The Women - 4:94
"Believers, be careful when you go out to fight in the way of Allah. Do not say to anyone who offers you peace, "You do not believe,"for the temporary benefit of this world´s life. For with Allah is much booty. You were like them before, but Allah has favored you. So search well and be sure. Allah is All-Knowing 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.