The Iron - 57:28
O you who have believed, honor Allah and listen to Him and believe in His messenger, so that He may give you two portions of His mercy, and grant you a light in the light of which you will walk, and forgive you. Allah is Forgiving, Merciful.
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.