The Repentance - 9:86
"When a Surah came down saying, "Believe in Allah and strive hard with His Messenger,"even the powerful and wealthy of them asked your permission and said, "Let us go and be with those who sit down."
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."