The Iron - 57:21
Run to forgiveness from your Lord and to a Paradise whose breadth is as the heavens and the earth. It is prepared for those who believe in Allah and His messenger. This is the Grace of Allah which He bestows upon whom He wills and/or whom He wills. Allah is the Owner of Great Grace.
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.