Something I saw in the comments which is an exciting idea - what if the church is a different person's dreamspace? Lalli has his cove with paths, Onni has his rock shelter and Reynir his grassy meadow with sheep. Reynir has demonstrated that he can accidentally enter other people's dreamspaces (including before he met the team, when he ran into Lalli's dream). The first time Reynir found himself in the pews, there was a voice asking if someone was there, and Christianity appears to have died out in Scandinavia, so then this could be someone outside the Known World.
Probably wrong, but wouldn't that be exciting?