Can you say "Holy pickup truck of St. Green"?
http://www.jimbutcheronline.com/bb/index.php/topic,28668.msg1235810.html#msg1235810The beast wasn't vulnerable to holy stuff, but it was worth the effort.
Basically, a true believer should be able to bless things, giving them the aspect "holy", at least for a while. Usually, those blessed things will be holy water or various symbols of faith, but depending on the believer, the range of objects might encompass all kinds of things. Including a pickup truck.
Depending on your campaign, the blessing might have to be more thorough, not just holy water, but holy water from St. Paul's Cathedral blessed during the Easter Sermon or something along those lines. But your garden variety monster should be vulnerable to your garden variety blessing. If it seems too powerful (like blessing the rain!), you could ask the player to spend a fate point for it.
Then, of course, there are things that will always be holy. The swords, for example. A believer might have imbued an ordinary weapon with the power of his faith, simply by using it a lot, and every attack with that weapon could be seen as satisfying the catch of "holy". Or the character has prepared things. Silver bullets cast from the materials of a blessed cross. A cross or even a bible verse carved into a bullet, doused in holy water and blessed by a priest.
Then there's the "holy" power. A martial arts true believer can be quite dangerous. I've also seen a power somewhere that allows a true believer to extend his holy power to the melee weapon he is carrying.
Exorcism example:
http://www.jimbutcheronline.com/bb/index.php/topic,28668.msg1921623.html#msg1921623