What level of security do "they" expect you to provide for your backpack and stuff? Protection against pilferage, or a more serious assault?
One thing to consider is an Exomesh outer mesh bag that can hold your soft-side stuff. It's stainless steel "chicken wire" with an aircraft-cable bag closure that you can padlock, and then secure around something immovable.

You can also buy backpacks, etc with Exomesh built in. They're slashproof and snatchproof, but you'd still need to secure them to something if you have to leave them unattended.
The Exomesh "baggie" is about $100, and rolls up to about the size of a softball when not in use.
Cool stuff. I knew you’d have something in mind.
I think a lot of it is a precautionary “lock of your valuables” kind of thing. There might be other residents or teams at the church, and we may not always know the group we’re working with on site.
Woah... that’s cool. Hadn’t seen that before. Gotta have.
Now THAT is just dang cool!