The Kingdom - 67:29 "Say, "He is the Most Merciful; in Him we believe and in Him we trust. You will know in the future who is indeed in complete error."
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."