How then should we understand the Crusades?
Jesus commands his disciples, “All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.” (Matthew 28:18-20) At no point does Jesus ever command us to promote the Gospel by force of arms. The Crusades were called and executed in the context of 700 years of conflation between ecclesiastical and political power. The Christian faith, which began as the faith of individuals who came to the faith and joined a community in the face of resistance from the governing authorities, had been reinvented as a community whose membership was mandatory, even on pain of death. In this context, and in the absence of access to the Scriptures by common people, it is not hard to imagine how people who cared deeply about such things as relics and holy places could be motivated to endure great hardship to carry the fight to Palestine. Such people, lacking sound leadership, could even be persuaded to commit heinous acts and think they were rendering service to God. All this is a valid criticism of Christendom, if by this you mean the church/state body that led much of medieval Europe. To use the crusades as a critique of Biblical Christianity is to attack a straw man.