This is what my friend asked me when I explained this manga's plot to her
i guess your friend and i have a kinda similar thought pattern
Probably like madoka magica kinda concept if you have watched it
ye i kinda did. but well the site got shutdown so i couldn't continue
SPOILER
They actually do have a way to determine saints! its said that saints have a regenerative power that as we know of, can't be found in anyone else. Elrise was just so strong that they practically never doubted that she was the saint and thus had no need to test whether she had regenerative powers. We even see Elrise hiding this flaw of hers in the manga. So there are actual saints. The witches have no way of detecting the saints as we know. This shows a bit in the fact that the witch grows afraid of Elrise' power and hides.
The saints are probably the strongest people around like, in the entire world, thus why they're the ones sent out to kill the witch. The "dark powers" from the witches apparently transfers to the saints that kill them and allow the saints to become the next witches. However, when the witch is killed by someone not a saint, the transfer of dark powers simply kills the person as they don't have the capacity of a saint needed to become a witch. This happens to Vernel in one of the routes in the game, where he kills the witch Alexia in Eterna's stead and dies for her. (Ch.9)
Thats also why Elrise' plan is to kill the witch herself, thus sacrificing herself in the process. I tried to explain to my understanding, The mention of Elrise sacrificing herself is in chapter 23 btw ٩(๑❛ᴗ❛๑)۶
sooo we all know about the fact that the world where elrise is has one saint and the other is a witch. but there is a witch and the original female lead is the true saint (according to what the mc thinks) but we have yet to see her powers in action. and what if the mc possesing elrise's body made her the actual saint? and even if that isn't true. she would end the cycle of the saints becoming witches. but shouldn't they be asking why the saints become witches? and what if it's not just the saints but the people that slay the witch? cause prob up till now the saint was the one to defeat the witch. tho i think the witch would know who the saint is as a former saint. they must have some way to sense a saint's energy or magic no?