I've done this is an english campaign... I had everything, refused to give the pope any land, got excommunicated about 5 times. As a catholic faction, he cannot be destroyed. Because as soon as you get a priest/bishop above 4 piety, he will go to the college of cardinals and thus be pope. and it is possible to assassinate pope's. I did it and ended up with a 95% chance of it happening. I killed off all his family, waited for my bishops to die, and the papacy still existed. I even wiped all of his forces into nothing.