[mivayan]

And multiple priests in a province have a cumulative effect (but still can't go beyond their individual limit of course). Just put 20 priest on that siege with the enemy god inside, and watch his dominion disappear from the province, even if you have a low Dominion score yourself...

Just a little note: According to the formulas posted by Kristoffer, holy2 priests cant affect enemy dominion of 8 or higher, you need lvl3 or better priests for that.