The Family Of Imran - 3:106
"On that Day some faces will be justified and some faces will be darkened. Those whose faces will be darkened will be told: "Did you disbelieve after you believed? Taste the torment for your disbelief."
The most important criterion of faith:
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.
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."