Let me get this straight...
Church is in need of a new sentinel, wait for any random person to need a place to live and get them in the apartment, allow the entities trying to escape hell to drive this person near madness, then swoop in and get them to take the mantle of gatekeeper at the last minute.
This is the church's plan.
Are they counting on these people being driven insane? Is it part of the process or is that just collateral damage? What if the person does what literally any person would do with all this batshit crazy stuff is going on and moves? Or is checked in to a mental hospital?
Why leave any of that to chance, even? Why not take a young priest or nun entering the service and train them for this one specific goal? They're taking some random person's life and ending it with this burden they didn't ask for when there's millions of people already out there willing to give their lives for god. There's nothing about this that makes any sense.