The shift from polytheism (which saw gods behind every part of daily life) to transcendent monotheism (which removed divinity from the affairs of man into some extra-temporal dimension beyond space-time) ultimately helped to undermine the theocratic basis of ancient society and enabled the political order to emerge as a sphere distinct from religion, grounded in the will and collective conscience of the people.
By the early sixteenth century, it had led Christian thinkers to utter statements like the following by a Spanish Jesuit intellectual of the Salamanca School:
"...No king or monarch has or has had directly from God or from divine institution a political principality, but by the medium of human will and institution..."
- Domingo de Soto (Defensio Fidei, Book III, 1613)
Such reasoning would have been impossible in any pre-axial age polytheistic society. Take Ancient Rome, where worship of the Emperor was the official state cult and the Emperor was the
Pontifex Maximus: the high priest of the Roman state, which was seen to be synonymous with the Roman polytheistic religion. Gods were everywhere - there were sea gods, land gods, gods of agriculture, war, peace, sex - you name it, the "gods" governed every facet of life. Religion was everywhere, embedded in society. The head of state was a priest-king whose legitimacy ultimately stemmed from his divine status.
The monotheism of Christianity completely undermined this worldview. God, through the Christian inheritance from Judaism, became in essence a transcendent "other" who was completely distinct from the earthly realm governed by men. All of the objects which ancient people had viewed as divinities were stripped of their sacral significance. When there's only one transcendent deity removed from the world, the entire world is thereby stripped of the sacred but equally opened up to new possibilities because man has freedom to chart his own path in, and with, it free from the "gods".
What's more, in the Christian religion God was thought to have become incarnated solely and uniquely in the son of a Jewish carpenter who had been executed as a condemned criminal - a man at the lowest level of the social ladder; a pauper riding about on a donkey without any political power. He alone was the Son of God.
This belief made it impossible to truly "deify" political rulers and the state in the way that ancient polytheistic peoples had with their priest-kings. The real and only Son of God, in the Christian mindset, hadn't had any earthly power at all.
Ideologically, the political order was stripped of any religious trappings or sacral underpinnings.
In time, this made it possible for secularism and atheism to emerge out of societies constructed on the basis of their monotheistic faith in an utterly "transcendent God" who had become physically manifested solely in a politically powerless Jewish peasant who had uttered the immortal words: "
Render to God what is God's and to Caesar what is Caesar's".
This was summed up well in the book entitled
The Disenchantment of the World by the sociologist Marcel Gauchet, who argued that monotheistic Christianity in particular should be viewed as "the religion of the end of religion" or the "religion for departing from religion" because it so thoroughly undermined the foundations of the ancient, polytheistic worldview in which there was no separation between church and state.