The Heights - 7:85
"And We sent to Median their brother Shu´ayb. He said: "O My people, worship Allah. You have no gods besides Him. A clear proof has come to you from your Lord. Measure and weigh exactly. Do not eat the people´s right. Do not make corruption in the earth after it has been leveled. This is better for you if you believe."
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.