The Apartments - 49:9
And if two groups of the believers fight against each other, reconcile them. If one of them attacks the other, fight the aggressor until he returns to the command of Allah. If they return, then settle their differences impartially and do justice. Allah loves those who do justice.
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.