Excommunication

Right, well, I've only ever been excommunicated once, and that was because I was bored, about to get wrecked and attacked the Papal States for a laugh. I'm gathering that if you hit zero crosses you will also be excommunicated? Or how does it work?

Be a Catholic faction. Declare war on another Catholic faction. As you attack their army and settlements you will loose favor with the pope. The pople will eventually, depending on your favor, demand you stop hostilities with other Catholic factions. Ignore this, usually once is enough, and you get excommunicated.

I had it happen to me a single time as England when I was fighting Scottland. I wanted to finish it up quickly so I ignored him. Conveniently the pope died shortly after that so I got reinstated a few turns later before I had a crusade called on me.