I believe crusades are limited to non-christian factions. I wanted one on Constantinople in the last game I played as HRE, to give a nice big fuck you to the byzantines with crusade units and a giant army of death with no upkeep, but it didn't show up on the list. I did manage a pretty good initial attack without it, hitting them on every front, while fending off... Austria, I think it was, who was allied with me, and decided to attack me (which I saw coming do to setting up watch towers on its borders). I seized their capital with the peacekeeper forces in my nearby cities, what units I could recruit in a couple of turns near the border, and a small army that was being sent to reinforce the front of the war on Venice and was passing nearby, before the Pope could bitch me out for defending myself against a suicidal fuckwit and demand I make peace with them or face excommunication. >:|
The pope died less than a year later from a bad case of crossbow bolt through the face. Truly a tragedy, and one his next few successors suffered, too. However, in times of trouble such as those, it is only to be expected that the Pope wants strong backing, such as that given by a strong nation, who controls most of the cardinals. Pope's died like flies but loved me because I put them on the throne.
I was doing well, but I got sick of playing Medieval II, and later uninstalled it to make room for ME2 and some other games, no room to install it now, and no patience to wait through the hour it takes to install.