I would say that that the dominance of religion in this age, was the result of the collaspe of empires and lack of political stability. At the fall of the Roman and Han empires, the west and east erupted into civil war. There were no dominant powers in either of the regions and therefore there was little stability. Religion provided a way to unite people from separate nations and regions under a common code. The western people adopted forms of Christianity to unite immensely different cultures, the people of Arabia united under Islam to fight persecution, and the people of Asia migrated predominantly to Buddhism to connect over a wide spread population. Religion provided a means of unity to a divided people of this age.

-King Arthur