The best answer is The Crusaders considered the journey to Jerusalem to be a kind of pilgrimage because participants would earn forgiveness for their sins.
The religious zeal that inspired Christians to wage war in God’s Name was common at the time of the Crusades. The threat of the Muslim invasion, the culture of violence coinciding with the teaching of Holy War, as well as the power and influence of the church and the pope during the time of the Crusades were all contributing factors to the reality and execution of the Crusades. Considering the fact that joining in a Crusade would mean forgiveness of sins and access to heaven, the importance of religious reasons cannot be overemphasized.
