The Apartments - 49:7
Know that the Messenger of Allah is among you. Had he followed you in most of the affairs, you would have been in distress. But Allah has endeared to you the faith and adorned it in your hearts, and He has made disbelief, evil and rebellion ugly to you. These are they who follow the truth.
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.