Something to keep in mind is that Christianity is presumptively monotheistic, even with the Trinity.
The Ancients were not ignorant of monotheism, but as there was no separation between culture and civics, religion and politics, monotheism equated with authoritarianism. As in one god, one ruler. "Divine right of kings," as it came to be known.
Populist forms of government, democracy and republicanism, evolved in pantheistic cultures, which were the Ancient's version of multiculturalism, as societies evolved from tribes to city states and territorial nations. Many gods, many voices.
When the West went back to public forms of government, it required a separation of church and state, culture and civics.
So there was a strong incentive for Constantine to adopt Christianity, as an imperial religion, with a reputation for benevolence.
The Trinity likely came out of the Greek year gods, which were symbolic of the cycles of the seasons, as was Jesus's death and resurrection.
A good book on the subject is Gilbert Murray's, The Five Stages of Greek Religion.
