Letters of Indulgence, Paying the Pope for your Sins.

You can play an agressive game vs other catholic factions, without getting excomunicated, as long as you have friendly relations with the Pope. Give him large cash sums or per turn payments to keep him off your back. Lol, you can even ally with the Papal States right in the begining to keep your relations high. You can now spend the rest of your game focusing on world conquest. This may not be an exploit, but to me it is kind of gamey. lol, I still do it most of my games.