The Repentance - 9:99
Among the Arabs are those who believe in Allah and the Last Day. And they consider what they spend as a means to draw near to Allah and support for the messenger. Indeed it is for them a means of approach. Allah will admit them into His mercy. Allah is Forgiving, Merciful.
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."