Obviously, anything holy or of Judeo-Christian significance is bound to hurt the evil creatures of the world, right? We see this all the time in media, even in instances where the legitimacy of religion is questioned. While this definitely can act on an (Western) audience’s preconceptions of how evil is defeated, it also runs the risk of saying things you might not mean and poorly fitting the story you’re trying to tell.
Why this is bad: If you want your evil creatures to be adversely affected by some kind of religious symbol or object, realize that you need to have rules for this, and those rules have implications. If you have demons that can be hurt by holy water – water blessed by a Catholic priest – but can’t be hurt by water blessed by any other religion’s priest, you’re implicitly saying that Catholicism is more correct than other religions. If you’re trying to create a story that doesn’t center around a specific religion (for example, Constantine revolves around Catholic lore, so it makes sense that this religions is shown to be more “correct”), then this can be problematic because you’re not showing major (or any) religions as equal. Not only can some readers get a bad taste in their mouth from this, but it also comes off as sloppy world-building.
How you can fix it: Define rules for your world and creatures. You do not have to limit yourself to the words in a Bible or Koran, especially if you’re trying to write something more universal. Take the tropes you’ve seen in the past (crosses burn vampires, demonic possession) and find ways to spin them anew. For example, in Death Defiant, demons can suffer severe burns from holy water. However, the world is much more universal, as I tried not to center it around a specific religion. Here, holy water isn’t defined as “water blessed by a priest,” but water that has been infused with a curse by angels specifically for offensive actions. By doing this, I’m still acting on some preconceptions that might be held by audience, but I’m also finding ways to work them into my story that doesn’t try to say that this or that religion is right, and it also works into the history and laws of the world I’ve built.
Bottom Line: Your world needs rules, so write them out. Dig deep into what can hurt “evil,” but also think greatly about why.