The Cow - 2:109
Many of the People of the Book desire, out of jealousy in their hearts, to turn you back to disbelief after your faith, even though the truth is clear to them. Forgive them and ignore them until Allah brings His command. Allah is all-powerful.
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.