It seems really willy nilly for now probrobly because we dont have a lot of backstory yet.
Only thing we know is that earth is their "7th garden" (did they have like 6 more before lol?) and that they're trying to cultivate their ideal garden by either nurturing or damning humanity with messages from "god" as angels O.o... We only recently discover from loki that they aren't just playing with humanity like what bell was doing cause she was bored and that they need the humans to grow the garden into their ideal thing and might be planning on wiping humanity out once they've achieved their new home or garden... thats just what I can gather from up to recent chapter on here though and I'm prolly missing a lot of info :'D
Kinda weird, no real explanation why the church and angels are intent on killing innocent people.