While more answers are always welcome, I wrote about this in response to a similar question yesterday.

For the first crusade, there is no doubt that religious fervor was a major motivator. Looking at the many chronicles, it looks like leaders, knights, clergy and lay people on the way to Jerusalem all felt that they were part a religious movement that was greater than anything narrated in the Old Testament. They were successful in spite of the odds, and they saw that as a sign of God being on their side. The chronicles are full of religious visions, miracles, relics. It really is an event centered on and certainly motivated by a great religious zeal.

Now, the first crusade is not the only crusade, and the crusading movement spans two centuries in the holy land and a longer time in crusading outside the holy land, including the Baltic. Motivation for the calling of other crusades varied, and there are several cases of crusades called by the pope for essentially political reasons. But if you are looking specifically to the crusade called by Urban II, there is no doubt of the religious motivation of the participants.

Sources: Riley-Smith, Jonathan. The first crusade and the idea of crusading. London: Continuum, 2012.

Housley, Norman. The later crusades, 1274-1580: from Lyons to Alcazar. New York: Oxford University Press, 2001.