Of course [SPOLIER ALERT] the notion that we (and not God) choose whether we view ourselves as devils or angels is offensive to many Christians, so revealing that plot point sooner might not have saved the show, but the show really does make one think and we do need more of that in this country.
Interestingly, as a Roman Catholic I was taught basically this. The fallen angels chose to separate themselves from God because they had perfect knowledge from their creation and are unable to be redeemed because they cannot unmake the decision, because of the perfect knowledge.
Us, on the other hand, have imperfect knowledge and we only get to make that decision after living our lives and being exposed to God at the final judgement. Again, however, we are choosing to separate ourselves from God because we find ourselves unworthy because of how we lived our lives in the light of revealed knowledge at the final judgement. That's also why Catholics are taught that people who lived their life outside of the knowledge of Christianity are not damned for that lack, but get the same choice once given the knowledge after death.
It's a difficult concept to grasp for some of the more fire-and-brimstone, God-as-punisher branches of Christianity and they tend to go on a bit about damnation and punishment in their preaching. The show seems to have gotten it right, but suffered because those branches don't understand it and find it offensive. Also, they might be offended because it points out that living a life going to church on a Sunday and being a bastard the other six days isn't going to save you once you have to reflect on that.