the fluid on the other side of the mirror

The other side is Hell.

The film doesn't bother to define Hell, but if you read the Bible then you can get some idea of it. I.e. there is lots of fire that never dies, worms that also don't die go around munching on people, demons hang out there 24/7, etc.

Catherine would drown and suffocate first if it was all watery fluid! She wouldn't even have time at all for the Anti God to even possess her!

It is another dimension that this being was exiled to. It is not hell as religion would describe it, and that is made clear in the dialogue of this film that hints that this being and the ones who banished it are aliens. Religion is a fraud and the secret society within the church knows the truth, hence the vial of liquid being hidden and guarded in the basement. Trying to apply the logic of how Catherine would drown in the water when it is some other dimension unlike anything man understands is rather absurd, don't you think? I mean, you do realize that the water was used to give the other side an ethereal effect and was not meant to be taken so literally, right?

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I am not a fan. I just happen to enjoy movies. Fans are embarrassing.