Romans 2:14-15 says:
"Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law, since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them."
From this I gather that the basic idea of God is something (according to the Christian Bible) that is a universal. It is something one can realize and follow without ever having read the Bible itself.
Because of this I believe that all religions are a common faith. The Buddhists, Hindu, Brahmists, Islams, ect. are all the manifestation of God's law written on our hearts into another form of religion.
"they are a law for themselves"
Is a statement that I believe can be applied to other religions. Through having the idea of God written on their hearts the created religions, a "law for themselves" so that they too may follow this idea of God.