Regarding crosses/vampires etc.
This is how I see it.
Their high concept allows Conviction based maneuvers, blocks or even attacks if you've got an implement of your faith (pentacle, cross, star of david, whatever), defended with monster discipline most likely, although perhaps sometimes some other attribute. You an also just skip the maneuver/block/attack and burn fate to invoke their high concept for immediate effect but that gives them the fate point to hose you with later, so if you're a high conviction fellow like Dresden or Father Fordhill, you'd most likely try the maneuver or block first.
Interpreted this way, the whole "you have to have faith for a cross to work" schtick makes perfect sense - low conviction folks by definition lack the kind of power to block a RCV with a cross the same way low discipline folks are unlikely to avoid being addicted by their venom, and low strength people unable to break their grasp. But if you burn fate and invoke the high concept, that's saying that in this one moment in time, you have enough faith to hold them off, even though most of the time a cross is just jewelry to you. (perhaps for the first time in your life you sincerely pray to a higher power for protection, or whatever, but it doesn't stick with you unless you buy aspects and/or conviction later at milestones)
I agree that some objects seem to "store" faith (holy water being the most obvious example), letting anybody screw with certain high concepts if they have an ally with faith. If you have a real "True Faith" person in the group, they might, for example, be able to bless the sword of the swashbuckler type, and have it beat holy item based catches, although mechanically it's likely handled by the faith person doing a conviction maneuver with a free tag passed to the swashbuckler, meaning you'll need to burn fate after the first free tag (or tags, depending on how the GM sets up the difficulty for a conviction maneuver of this sort...might work until the scene is over to beat catch, but you can only invoke for +2 or reroll once without fate...)