Prophet Hud - 11:40
"Finally, when Our command came and the sky boiled and overflowed, We said to him: "Load on him a pair of every kind, his children and those who believe, except those who were condemned before."Those who believed with him were already a few."
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.