What does being a saint mean?
As the power of Saints is based on the Idol Theory (humans are replicas of God so they can reproduce a fraction of his power - Saints are just humans who are "better replicas" in this sense), it seems to be something innate and defined at birth. Brunhild's backstory in the Kanzaki SS also confirms the Saint status is something one is born with.


  It's actually a core plot point in her story since she suffered heavy discrimination for it.