This is best used when neighbours of the target's faith are engaged in other wars and/or the target has low Piety. Note that if you use this CB, neighbouring rulers of the defenders faith will be able to (and frequently will) intervene in the war and the defender will be able to summon Holy Orders for only the initial Piety cost. Since patch 1.091, this CB may only be used on territory bordering yours or within two sea zones of yours.
The winner's religion gains 3% moral authority for 20 years and the loser's loses 3%. If you stalemate you'll lose 50 piety and 100 prestige, and if you lose you'll lose 150 piety, 200 prestige, and be forced to pay concessions. Upon victory you'll gain 50 prestige and all holdings (as long as they're occupied by you, or unoccupied) in the targeted duchy. Muslims can also attack non-heretic Muslims of a different denomination (Shias if Sunni, and vice versa). If you're in a different religious group than your target, your target is a heretic (of your parent religion, so Catholics can't attack Monophysites as they're a Miaphysite heresy), or you are heretic, you can declare a religious war. Ive just copied this from the a wiki page, hope this is still correct and helps:
