The crusades were motivated by religion. Every crusade was about capturing Jerusalem, Which is meant to be the most holy place on Earth. People went on crusades because they wanted to capture Jerusalem so that they could be closer to God, Or that they were promised a place in heaven, Or because "God wills it". All of these reasons to go on crusade are to do with religion. Also, The title "Martyr" was given to those who wore the crusader cross and died in combat. Most people believed that important to go on a crusade, Most people also knew that is was good to be standing with the church in the time of the crusades. That is also a reason why people might have wanted to go on crusade.
The Catholic Church at this time taught that anyone who went on Crusade would have their sins forgiven and would go to heaven. We can assume most people were motivated by this due to the 70% or so casualty rate. Not to mention, there really wasn't much plunder to be found in the Holy Land at this time. Sadly the people of the time had the bible kept out of their hands and didn't know what true Christianity was about. Thank you and God bless you all.
I believe the majority of the people who volunteered and joined the Crusaders were doing it to gain favor with the Church, while I believe most did honestly believe in the goal of Crusade I believe that most knew it was very important to be in good standing with the Church back in that time period.
The crusades became about other things like land grabs for the sake of land grabs and building empires in the interest purely of fame, but without religion they would have never happened to begin with. Religion motivated the crusades to happen and was what fueled them to noteworthy levels throughout.
It is my opinion that the religious Crusades were not primarily motivated by religious piety. I feel that the resource of land was the primary motivation for each side in this religious conflict. Religious piety played a lesser role than gaining land in the Crusades, and functioned as a secondary source of motivation.
War is almost never started purely due to religion, and the main underlying factor is nearly always money. The fact of the matter is that during the Crusades Europe was struggling economically, and there was not much of anything to go around. The Crusades thinned out the population allowing there to be more to go around.