>>718886461
Either Alvin or Toriel know the Delta Rune code
If Carol (the mastermind) actually wants Toriel captured and hasn't just been using her as bait, then having The Knight wait for her to show up for choir practice at the Church makes sense. Toriel fails to show up because of the rain, but we know from the last Chapter that The Knight is not particularly eager to capture her anyway, and is happy to just fight with the Heroes and keep them busy (they're obligated to "duel" according to the Prophecy).
If Alvin was keeping the Delta Rune code, he's probably already been kidnapped like Undyne and we won't find out until Chapter 5. The Church being wrecked at the end of the Chapter could have been signs of a struggle when The Knight physically came and took Alvin away in the Light World. If Alvin had the code and is at least somewhat aware about the Dark Worlds and that he might be in danger, that would explain why he was keeping Gerson's remains in his desk. The first fountain in the Church entrance, which brought Gerson to life and is inexplicably depicted as more thin than any of the other fountains we've seen, might have been created by Alvin in an attempt to defend himself against The Knight (or summon Gerson to aid the Heroes).
In this scenario The Knight sticks around in the Church for similar reasons: their objective has already been accomplished, and now they can distract the Heroes from what's really going on, waste their time, and test their strength for the future.

>>715636195
He's not too important, but not too unimportant.

>>715636406
It makes sense to me.
Toriel was never the target, she was bait to lure in Undyne in Chapter 3 and she was bait to lure you into the Church after it was already too late to save Alvin in Chapter 4.
The first Fountain in Chapter 4 is inexplicably smaller than the others, and Ralsei says that making a Darkner from "the memory of a loved one" would only be possible in a "very specific" Dark World.
Alvin is nowhere to be found at the Church, even though there was supposed to be choir practice and the Church holds services at night.
The Delta Rune symbol most clearly points to The Church, and Alvin sternly warns Kris away from the Shelter if you ask him about it.
And the Church is the closest building in town to The Shelter. If The Knight was going to take action in broad daylight, making the run from the Shelter to the Church would be where they'd do it.