Most certainly, religion was used by the Pope and the kings and princes of Europe to motivate the soldiers they needed to mount their crusades on the Holy Land. But given what has come forward as the conditions of Europe in those days, the idea that it was religion alone that drove the engines of the Crusade seems to be an over-simplification and, as far as the kings, princes and the Pope were concerned, seems to have been the vehicle for more secular (or venal) motivations. Why, for example, sack Constantinople in one of the crusades instead of proceeding to the Holy Lands as originally planned (4th Crusade in 1204)? Because the odds of success were too low and the Byzantine city was easier pickings. What may have started off being a religiously inspired cause quickly became more a matter of redirecting and marshalling large bands of armed men who could cause trouble at home and sending them out to grab sources of wealth outside of their home territories. It could be language, race, ethnic origin, or, as you say, religion. It is how one group may self-identify with another. In many cases, all of these elements and more are bound together as one package which I am more accustomed to seeing as being a "national" identity.