@finland
Then it's still not PvP, since there wouldn't be any real fighting at any point. To fight, you need an attacking team and a defending team. With no defending team, it's just attackers spending five minutes to get the buff and move on, only for attackers from the other team to get it back in five minutes. Which is possible, sure, it just seems mind-numbingly boring to me to have a series of completely one-sided battles. There'd be no risk for the attackers at all unless they stub their toes on the way there.
@gothix
It would definitely force guildmates against each other, true. That sounds like an interesting thing to see actually, but it might be irritating for the players in some cases, and some would for sure decide to just follow the same god to make things simpler.
For those reasons, I'm personally all for making this GvG, but if the third option is Good vs Evil, I'd accept Religion vs Religion as a compromise.