The Ornaments of Gold - 43:33
Had it not been that all mankind would have been one people (in disbelief), We would have furnished the houses of those who disbelieve in the Most Merciful with silver ceilings and stairs for them to climb up.
The most important criterion for believing in Allah:
The Cattle/6:82
Those who believe and do not mix their faith with wrongdoing, they deserve security and they have found the truth.
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.
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."