I just finished binge-reading everything over the last 5 days.
This manga is wild. I love how many curveball it throws, how much foreshadowing it actually uses, and how many twists it has.
So, if I understand right, the Holy Spirit is essentially a symbiotic parasite of humanity. It gives magic but in exchange feed off human conflict with said magic and control things from the shadows.
I'm pretty sure they actually reincarnated everyone in modern Japan with the goal of a mass killing happening with the forbidden curse so they would "spread" over more uninvolved normal people. After all, they can reincarnate anyone hit with that magic. It would only be a matter of time until magic control our world as well. That is, unless our MCs manage to destroy the Holy Spirit as planned, which is now the end goal of this manga.
It's also very interesting how Allen keep getting involved in so many of the characters backstory. He doesn't seem to be a nefarious person either. I think his goals were actually aligned with our MCs, in that he wanted to destroy the church from the inside.