Something odd is going on, biologically-speaking. Those plants on the pews...I imagine they were once people. The wildlife also seem like they have mutated. And the way the plant life has interacted with this building and the rest of the town... All of it is likely the change that this person speaks of.

[This sure is one way to cope with it, though. Believing this to be their salvation.]

Was there anything on the altar itself, Rean?