The Cow - 2:256 There is no compulsion in religion. So the truth is separated from error. Whoever disbelieves in Taghut (despots and evil doctrines) and believes in Allah, he has clung to an unbreakable and firm bond. Allah Hears, Knows.
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.