Just like the title, can some summarize the past arc. Every time I try to read it, I grit my teeth and want to break the screen. It's too painful.
Past world was a modern world a bit more advanced than ours. Except there were dragons. And a stray angel Sariel, whose mission was to protect weaker life forms, and who became attached to humans, and prevented dragons from messing with humans. In the past, direct interventions where she directly used her powers to help humans tended to have bad outcomes, so instead she set up a charity organization for humans and didn't help by using her powers (so most humans didn't know what she was). At some point she picks up the children who were the results of Potimas's experiments, including Sariel. Gyuri was a young dragon who got an existencial crisis upon meeting Sariel and realizing that dragons weren't the most supreme beings. He stalked her for a while, gained a crush on her, and got a bit more interested in humans because of her. There was a human scumbag called Potimas who wanted to be immortal. He did human experiments, and studied the dragons. He discovered the existence of MA Energy and made it public for his own purposes. Humans started exploiting said energy without realizing what it was, despite warnings from the dragons and Sariel. Only Justin (future pope), the president of a country, opposed its use in his country. Well the dragons couldn't kill off the humans because of Sariel, so they went 'screw it', took the rest of the energy themselves and left. The rest of the humans finally realize they screwed up. Justin is now considered the wisest guy ever. Potimas came up with a plan to decompose Sariel to make up for the lost energy. In fact he plans to just suck up said energy right after. The world leaders didn't know his plan, and Justin (who had to make the final decision) reluctantly agreed. Gyuri doesn't want Sariel to die so he goes begging D, a big-shot god that normally nobody dares to mess with, to save her. She agrees because it's interesting, but what she actually does is set up the system using Sariel as the core. Humans need to kill to grow their souls, and their extra power is reverted to the system to fix the planet when they die. Sariel being Sariel, she begs Gyuri not to kill humans, so he couldn't kill Potimas either, so he keeps doing his stuff. Evolved humans eventually appear (demons), the ex-experiment children also scatter, eventually normal humans start persecuting the evolved humans and that's how we got the whole demons vs humans thing. Justin felt like a scumbag when he agreed to sacrifice Sariel, which is why there's that whole religion thing.