The Repentance - 9:38
"O you who believe, what has happened to you that when you were told, "Take action in the way of Allah,"you fell to the ground? Are you content with the life of this world instead of the Hereafter? The blessings of the life of this world are little compared to the life of the Hereafter."
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.