The Pope in my game was pretty quiet until Sicily attacked him in Rome. He then went berserk, taking Corsica and demanding first that I crusade against Rome, and then that I give him Rome. He proceeded to take the rest of Italy, and then went after north Africa and he also owned Jerusalem for a while until the Timurids booted him out. He's still in the Holy Land with a half-stack, though, apparently waiting for a chance to take Jerusalem back.