@Ozzykun Kinda forgot about this series for awhile, so I'm just catching up, but I'd wager your first guess is correct (though you probably already know the answer). Most fiction featuring heaven and hell as literal forces generally have them bound by some kind of pact of non-interference; neither interferes with the other while also not directly interfering with Earth. So bringing holy artifacts into hell would have counted as a violation of that pact. It could also be the intent, since heaven and hell are big on those kinds of things. He intended to bring holy weapons to hell for the explicit purpose of defeating demons.
The bigger question, though, is why
Shinto artifacts would break a covenant between two
Christian forces.