How did belief become equated with religion?

​Question:  How did belief come to be equated with religion in the first place?  According to John Gray in SEVEN TYPES OF ATHEISM, our earliest religions had nothing to do with belief.  They simply offered myths that entertained and explained (though not in the modern sense) and practices to be followed (burnt offerings, observance of rites, rituals, and celebrations, etc.)  Beliefs weren't talked about or thought about at all.  And Buddha, Confucius, and some others never spoke of beliefs at all.
 
Response:  It probably all goes back to the Apostle Paul.  Instead of following the teachings of Jesus, which were basically about observing the Law of God as He understood it, Paul repeatedly wrote that one can only gain admittance into the Kingdom of God by believing in the death and resurrection of Jesus—a claim Jesus never made for Himself.  For Paul, however, this was the essential matter.  As a result, much of the human world has now become infected with the pernicious notion that belief is not only imperative for human life, but necessarily good.