Religion was the main reason for the war of the Crusades, religious gain.
IS is nothing different, and yes, it is their main reason for -can I can it war? -in our time.
All who die by the way, whether by land or by sea, or in battle against the [Muslims], shall have immediate [forgiveness] of sins.
-Pope Urban II, 1095
At the urging of Pope Urban II in 1095, the First Crusade succeeded in taking Jerusalem and was the most successful from the European point of view. When Jerusalem fell in 1099, crusaders massacred Jews, Christians and Muslims alike. Then the leaders divided up the land into territories, each governed by a European feudal lord. - First Crusades, 1095-1099
The Second Crusade started when Europeans lost control of Edessa, territory that they had previously controlled, to the Muslims. Led by King Louis VII of France and King Conrad III of Germany, the Europeans failed to regain any land and the crusade was a failure from a European point of view. 1147-1149
The Third Crusade was a response to Jerusalem’s fall in 1187 to Salah al-Din (Saladin). Three great armies from Europe were led by Richard the Lion-hearted of England, Philip II of France, and the Holy Roman emperor Frederick Barbarossa, Europe’s greatest warrior. However, Frederick accidentally drowned, the other two kings quarreled, and the whole crusade failed from a European point of view. 1189-1192
Instead of going to Jerusalem, the Fourth Crusade went to the Byzantine Empire’s capital, Constantinople, for various reasons. The European armies looted (robbed) the city and overthrew the Byzantine emperor. The main outcomes of the Fourth Crusade were to deepen the division between Greek and Latin Christianity and to hasten the Byzantine Empire’s decline. 1201-1204
When you study ancient European history, and the Crusades it's impossible not to connect economic woe to a Crusade. Can only tax your own people so much before you're not able to sustain yourself on just that. But hey, if we go to the mdidle east they got lots of loot.
There were multiple reasons but the biggest was always money. If you didn't need money, you probably fell for the remission of sins thing. If neither, hey you get to kill all the pople ya want without penalty. Probably appealing to people of that time as much as loot.