There wouldn't be any True Believers left. They' believe on purpose to banish the demons which wouldn't make them a true believer. It's confusing.

Propaganda? Erect many speakers around the known 'walls' and play the verses in the Bible on ultrasonic freq, then if possible try to domesticate one of them starting with those already possessed and then aim to conquer the other dimensions beyond the 'walls'. Show them who's the demon!

Belief based on evidence counts as true belief in my book!

Check out a movie called Constantine

Your rules seem to allow for a new "demon hunter" religion. It is based around the idea that demons are inherently evil and need to be banished (B), the practitioners create methods of banishing demons (A) and in such a world, it wouldn't require much to get the "believers" you need (C). Is it your intent to allow such a thing? If so, that would greatly reduce the power of the church. If not, what is the factor that disqualifies such a "religion"?

What prevents the church from turning the whole world into "holy ground"?

So, what you are asking is this: How powerful will a church become in a setting that demands churches to become powerful?

I find this question related: worldbuilding.stackexchange.com/questions/34208/…

Just a thought: all the answers below seem to be referring to the Christian church(es). But there are lots of other religions, and most of them have demons. Those religions might also see an uptick in believers/importance as people who for whatever reason don't go to the Christian churches go to them for their spiritual/anti-demon needs. So your churches might not become quite as powerful as other answers suggest.

Are demons intelligent? What is their character? Objectives? Is demon/spirit/angel a subjective distinction? Can people be confused between angels and demons? Can people commune with demons and use them? What about religions which worship or commune with spirits? Can demons be turned into something else (like energy; Doom 2016 plot right there)? Are demons scifi enough to be scientifically explained somehow as interdimensional beings?

I've just created a new religion, everyone is welcome, as long as you perform the breathing ritual. Oh, and I'm the messiah who banished all demons from earth, I'm using my status to consecrate the whole universe.

@AlexandervonWernherr That would only be true for some of them, but many people would become believers if you performed true miracles in front of them like exorcism. There is a reason Religion is still massively prevalent in modern society despite the lack of obvious miracles. True miracles would solidify those beliefs like concrete, and their children would be born and raised as believers, who would witness these miracles first hand, making it impossible for them to ever doubt the religions validity.

In what way are the demon's malevolent? Why would they attack humans? A pack of demons cutting down a random jungle in Africa and burning isn't real serious to most people (at least, no worse than a random forest fire) demons descending on Chicago and obliterating it is a far more serious matter. Are they malevolent towards man or malevolent towards all life on Earth? If so, I think man might see a lot of ecological issues spike in intensity, but maybe not see a lot of direct attacks. Churches might just do more ecological activities to prevent general deforestation.

Well......If they are unkillable, it would show just how powerless the religious orders really are, therefore membership in them would drop considerably. But it is true that there are both good and bad demons. If we go by Christian/Judaic doctrine, there are only bad demons, but we all know there are MANY more religions than just those two.

One thing though, as a lot of religious groups have slivers of corruption in their cores, take catholicism over its history and some of it's cover ups, and the protestant sects as well, and said corruption/malice can open doorways for demons......we are all screwed.

The scenario reminds me of some Terry Pratchett novels.

See also: this question here.

There are some holes in this question. First, would you need to believe in a religion to be able to fight demons, or simply in demons? If I believe with all my being that I can banish demons will that not be enough? Second, are some demons friendly to humanity? Or at least not actively hostile? Is there anything to be gained by dealing with them, or are they all hell bent (pardon the pun) on destroying mankind?

If any religion can banish demons with their own rituals to their own gods, then did all religions become right at the same time?

@user6760 You might be interested in the web novel Pantheocide

Are demons unable to enter ground considered holy by a group that worships them, e.g. Satanists?

if demons are regularly visiting and causing mayhem, it won't take much to be a true believer, the evidence is there for all to see.