The origin of the church was the witches who once ruled the world of mankind. The first popes where especially powerful witches. It was decided by a witch that to fight the demons, witches had to be sacrificed to make "God's Punishment Warriors" which required the blood of witches combined with the "stone of god's retribution". The person who underwent this process lost their personality entirely - they are just mindless drones that do what they are commanded to do but they are very strong. The process often failed too, in which case they also died - or rather their body as you could say their mind/personality dies either way. If everything "goes well" (with as much as they've refined the process in the last 400 years), 1 witch's blood can get you 2 God's Punishment Warriors. For all that sacrifice, "as long as humanity survives, new witches will continue to be born." It is a strategy born of desperation.