The same could be said of cold iron and the Fae. That one's on wikipedia. And yet, it's only a +3 Catch (+1 from knowledge, +2 from being common as dirt). That +2 from knowledge is really stingily given out in the default universe.
I'd argue there's a pretty big difference.
Having to look it up on wikipedia is significantly
less well known than being such a significant part of popular culture. Assuming that you're setting your game in the English-speaking parts of the West, a good amount of knowledge of Christianity (or more specifically, Catholicism, I suppose) should be taken for granted. It's such a core, underlying part of the culture that it's pretty hard to have a good cultural awareness without picking up things, even if it's just watching the Exorcist, or reading Hellblazer.
That said, I
strongly disagree with the Fae only getting +1 for knowledge. It's explicitly stated in the rulebooks that +1 is for when it "requires
access to specific research material that could be restricted (like a wizard’s library)". Wikipedia doesn't fit that at all, nor does the
Fairy Lore Handbook, by Ashliman, or
Faeries, by Froud. Both books, by the way, mention the weakness to iron, and Amazon could have them delivered within the next 24 hours or so. Frankly, I don't think it could get much easier for me to find out, short of having it as general cultural knowledge that I'd pick up naturally.
The idea that they should be getting +4, but it's capping out makes a lot more sense to me.