The Cow - 2:186 And if My servants ask you about Me, let them know that I am near, and I answer the call of him who calls to Me. They should respond to me and believe in me that they may find the right path.
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.