This story is truly magnificent. I doubt I have ever seen such a manga written so wisely as such an explicit way.
It certainly challenged many of the assumptions that many other manga hold. The idea of the goddess is a great example. Most manga regard religion as something like "superstition", "for the weak". But this manga show that there is more to it than meets the eye. That's just one of the many examples of the profundity of this story.
Another idea that might seems controversial, the idea that all demons are evil. They use words as tools to deceive and hurt other people. Some well-meaning people might accuse that as some kind of "racism", saying that you cannot demonize all of the demons merely because they belong to that particular "race".
But if history teaches us anything, it teaches us that self-delusion in the face of unpleasant facts is folly. Demons are different from human beings. They are not sentient. They have no emotion, no sympathy for the others. Their only goal - is the indefinite expansion of their power, and they would use anything, do anything to achieve that purpose. They have no morality, only status and power.
While some would come to believe that we could rehabilitate those creatures through "love" and "compassion", just like any other fairy tales... They don't understand the evil that exist in our world - evil is not something that cannot be defeated through appeasement