The question I want to ask: Why did they use a metapher for a metaphor in that story? Wouldn't it have been easier to just use demons in the first place?
It certainly wouldn't have hurt the story, and it removed most of the "internally stupid" things of the movie.
Hey, it could even have worked with the aliens, if they just let Mel "bless" the water or something like that. Though it still doesn't make sense to use the crop circles.
But, well, let's be fair, a bit of the so-called "fan-wanking" might be in order:
1) Water hurts alien:
Indeed, the water is blessed by the ex-priest (and possibly all other peoples that found their belief during the attack)
2) Crop Circles used as landing help:
Actually, they aren't. They are just a warning from god. The aliens just notice the unusual markings and consider analysing it (maybe they are even familiar to them, because this isn't the first time they fight against gods children, be it on Earth or anywhere else)
3) Alien equipment:
Uh, I am a bit at loss here. Maybe they weren't naked, their skin was actually body armor (maybe like Shadowrun Dermal Plating). They didn't use ranged weapons because ... god disabled them?
Oh, I have seen and done better fanwanking. Maybe that's a sign that it doesn't work for this movie... Maybe I am just not trying hard enough, because I really didn't like these weak spots of the movie. (The rest of the storyline might have been okay)