[Gregstrom]

P94 of the manual gives a preaching formula. It looks like H1 priests aren't worth much when preaching. H2 priests are good workhorses for preaching duty, and H3 are excellent. H4-5 will counter enemy dominion effectively, although dominion change in this way may not be an effective use of their time.

Sombre is right about the priest level cap on preaching dominion, so don't expect multiple H1 priests to give much of an effect.