CC - Schism between the replaced religions and the new Christian ways for a populace was quite necessary not to totally alienate them. Especially in the event of a conquered territory where the people needed to turn to their original faith to help them through the political and social transitions. Our modern concept and traditions associated with Christmas are excellent examples.
I believe that without the heavy political influence of the Vatican over the centuries Christianity would not be as, for lack of a better word, “divided” into various groups (Catholics, Protestants, etc.). I’m not suggesting that it would still be just one group, but far fewer than now in comparison.
I believe history saw a “shift” to monotheism to better accommodate period laws and monarchs. Multiple Gods worked well for the city-states in Greece, throw-backs from more ancient beliefs. Later, unifying countries under one monarch-one God with real law codes was more feasible (increases in trade made a lot of this necessary). Kings wanted a piece of it all, especially to pay for the armies keeping them where they were…