Lesson of the ancient Roman Empire: Although they accepted the idea of one god/one cesar they were polytheists at the same time. (?) ..
It is to be considered that the people of ancient greece and rome were religious polytheists, monotheism was only a matter of philosophy (science) not of
daily life and religion. So the idea that there is only one divine principle was known but had no effect in religion. But the Emporer August was clever enough to called himself God. So he had absolute power.
The decline of prosperity, security and unity of Empire was clearly accompanied by loss of faith in Rome's traditional gods and – at least in the West – in Roman emperors. For some Romans, this was caused by the neglect of traditional religious practices. For others – equally Roman – breakdown of empire was God's judgment on faithless or heretical Christians and hardened pagans alike.
As Roman society evolved, so did cult to emperors: both proved remarkably resilient and adaptable. Until its confrontation by fully developed Christian orthodoxy, "Imperial cult" needed no systematic or coherent theology. Its part in Rome's continued success was probably sufficient to justify, sanctify and "explain" it to most Romans.[259][260] Confronted with crisis in Empire, Constantine matched the Augustan achievement by absorbing Christian monotheism into the Imperial hierarchy. Cult to emperors was not so much abolished or abandoned as transformed out of recognition.
https://en.wikipedia.org/wiki/Imperial_cult_(ancient_Rome)
Follow the google book
Violence in Ancient Christianity: Victims and Perpetrators
https://books.google.at/books?id=OU_3AwAAQBAJ&pg=PA67&lpg=PA67&dq=polyarchie+polytheism&source=bl&ots=ikQ8XQKpWe&sig=028RRicwkuK9xtSGZGIAKM1g7FY&hl=de&sa=X&ved=0ahUKEwjP9Zr9qrrQAhUKthoKHbvVAawQ6AEISjAI#v=onepage&q=polyarchie%20polytheism&f=false