The Cow - 2:76
"When they meet the believers, they say: "We believe"; but when they are alone with them, they say: "Do you tell them what Allah has revealed to you so that they may use it as evidence against you in the sight of your Lord?"
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.