I think this might be possible with the SDK. I'm not sure what the particulars for the victory condition should be, but I am thinking your religion(s) have been adopted by 75% of the world population.

(I founded 3 my first game and determined that 66% would be too easy.)

You'd need some way of removing the influence of a certain religion in a city, then. Otherwise it would just be a matter of sending missionaries into most cities - much too easy.

Hmm .. inquisitor units

Yeah, I don't understand why I don't have the opportunity to commit some real world class human rights abuses by persecuting unpopular religions and forcing the conversion of my own population to a religion of my choice....

I think the best means of getting a religious victory would be to get all other civilizations to convert to your faith and have X amount of the populations (not the cities) worshipping your God.