You know sometimes the pope ask for a crusade against a faction? You can start a fresh crusade for free if you have a "cross" right? You can actually have two concurrent crusades, if you have have one already going to (or start a fresh one against) another faction, you can then have another free one at the pope's request. But make sure you start your own one first. The Spanish AI is religious, so it normally have spare "crosses" ready.