I must give whoever wrote this points for effort. Being a christian myself, I do support spreading the word, but I think they should have tried something a bit more believable. Even if the Dursleys did become Christians, the transformation wouldn't be overnight. It would be like trying to quit smoking. No one does it all at once, it takes deliberation and willpower. And wizardry isn't a religious manner so I don't see why Harry would have to renounce it. Wicca, maybe, but wizardry (bookwise) no.