The Coalition - 33:19
They are stingy in their dealings with you. When danger comes, you see them looking at you with their eyes wide with fear, like one who has fallen into a faint of death. And when the danger passes, they hurt you with sharp tongues, being fond of wealth. Such people are not believers. That is why Allah makes their deeds vain. This is easy for Allah.
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."