The Cow - 2:143
So We made you a clear-minded people so that you may be witnesses among the people and the Messenger may be a witness among you. And We changed the Qiblah, which you used to turn towards, to distinguish between those who follow the Messenger and those who turn back on their heels. Surely this is too much for anyone except those whom Allah guides. Allah will not make your faith vain. Allah is Compassionate to mankind, Most Merciful.
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.