I think a hit on this link ought to answer that question.
As far as Hillary possessed..you might find this to be an interesting read...
J6P
Surprised it wasn't Extra, Extra Virgin Olive oil??? I've been told the holy water is just ordinary tap water that's had the hell boiled out of it.
My impression is that the power comes from Christ and garnered via the faith of others, but nonetheless He is the power which gives reason for fallen spirits to depart from possessions of other persons. The issue isn't as superstitious, as much as it is supernatural.