Since all social orders and hierarchies are imagined, they are all fragile, and the larger the society the more fragile it is. The crucial historical role of religion has been to give superhuman legitimacy to these fragile structures, Religions assert that our laws are not the result of human caprice, but are ordained by an absolute and supreme authority.
In order to unite under its aegis a large expanse of territory inhabited by disparate groups of human beings, a religion must possess to further qualities [the previous two are insisting that there is a superhuman order that was not invented by humans and establishing norms and values that are binding]. First, it must espouse a universal superhuman order that is true always and everywhere. Second, it must insist on spreading this belief to everyone. In other words, it must be universal and missionary.
As far as we know, universal and missionary religions began to appear only in the first millennium BC. Their emergence was one of the most important revolutions in history, and made a vital contribution to the unification of humankind, much like the emergence of universal empires and universal money.