In Toaru Majutsu no Index, there are currently about 20 saints
as of before the Acqua of the Back-Arc.
So far I've read until the arc named in the spoiler and I'm wondering, how you become a saint and what makes a saint?
Kanzaki mentions that she was born with that ability, so it's most likely that it's a trait that one has all his life. But it seemed to me, that Acqua of the Back wasn't a saint back when he worked for England; Is this wrong or did he become a saint when he became a part of Gods right Seat?
It also seems, that a life of a saint doesn't depend on being a saint, because Acqua comes back later.
Are saints just extremely powerful? Or do they have another special trait?