The Repentance - 9:122
It is not right for the believers (who remained in Makkah) to set out en masse (to visit the Prophet who emigrated to Medina). Only a few from each group should set out to understand and learn the religion. That way, when they return to their people, they can warn them so that their people will be vigilant.
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.