The idea is fantastic, but I dont like any gody interferance in fics, because mostly its just a lame way to create a situation without developing reason and motivation for the characters.
But how about the idea that lily's sacrifice manifests in some kind of shine and from there you go.
You could make the blood protection really strong by Harry saving Dudley somehow with his accidental magic and then petunia starts loving him, thus its strongly there and gives him a 'holy' power. Than Petunia gives him to the church in pride, so you don't have to deal with the Dursleys any longer.

 

Oh no you don't.

You're going to finish your own stories first.

 

No, don't give in to peer preasure jon! Write that fic!

[size=-2]Omg! A squirl, its peer preasure!!!![/size]

 

Meh, you don't need God to actually exist. The only appeal of this idea, to me, is the fact that the Vatican has mounds of their own spellbooks, perhaps their own form of magic. That's what makes me like this...of course, having Harry do thing in the name of God can get squicky, but people over the ages have done all sorts of things in the name of god. It doesn't have to be squeaky clean, it could go so far as an ethnic cleansing :/

 

If anyone has read any of the Dresden Files series, then there is a bit of similarity between this idea and the Knights of the Sword.

 

When I read this challenge, I immediately saw Harry as a man along the lines of the Boondock Saints, but not rogue vagabonds in the way that the characters are in the movie... sort of. I saw him more as a saint of sorts, he fights for the church, in the name of the church, quoting bible verses and fighting a neverending war against the corrupt.