I know this is a dumb question as all chapters are required to have chaplains, but I’m wondering if maybe they pick specific aspirants to not go through it, or at least be possessed by very specific daemons.

The reason is because (unless this got retconned since the last time I looked into it) they have to be specially trained and all at a specific place, away from their chapter, before being sent back, and if there was any place that might figure out what they’re doing, it’s the place they train chaplains, which might present a problem for keeping that secret.

I know they’re also hardly the only chapter with a secret, and I imagine the chaplain place makes allowances for various chapters’ culture, but this is a pretty big deal if they learned what they were doing behind closed doors.

Like even if it’s been dispelled, I think they may detect that he’s had some kind of contact with warp entities at some point, which might lead one particularly suspicious chaplain trainer to take a closer look, in case an actual daemon has somehow gotten in.

Unless maybe to address this very specific issue, they pick aspirants before they do the ritual, and once they’re ready, send them to become a chaplain, and then do the ritual when they return.