Scion said:Point is, it is easy for the people to get past it who it is supposed to be used against.
A mantle of faith is "supposed" to be used against villains. Villains in D&D span the gamut from your lowly 1HD orc warrior, to demiliches. Of these, I say again, the vast majority will not be able to penetrate its DR. Even if you limit yourself to "enemies of the faith", the vast majority of those still won't be able to do it. You have this strange impression that you're only supposed to use a mantle of faith against demons, when there are tons of other worthwhile foes around.
The same holds for aligned outsiders in general. Unless your campaign centres on a huge, all-encompassing planar war, there's no reason to suppose that angels spend all their time beating on demons, and vice-versa; there are plenty of other horrible evil beasties to get their attention.
If you had a device that went around and whenever you placed it over a persons head, asked them if they commited murder last night, and it killed them if they did not commit the crime you could keep going until you found the murderer.
Of course in the process you have killed lots of innocent people, or at least innocent of that crime.
Same thing, different name.
Arguing by analogy is the second-lowest form of wit.