Sucesss against Islamic factions and high zeal? Spain and France are very zealous.
Or maybe sucessful crusading - since the most crusading factions tend to be Spain, France, England. Hungary and Poland can't in vanilla and the HRE usually don't. Italy is normally pissing off the Papacy. Sicily generally don't crusade either (I'm not sure if they can or not).

Hopefully it will be more established in MTW2