The primary purpose of the Crusades, according to Barbara W. Tuchman's book "Bible and Sword: England and Palestine from the Bronze Age to Balfour," was to serve as a cleansing force for Christendom. The phrase "the sewer of Christendom" implies that the Crusades aimed to drain away the various conflicts and discord that plagued Christian society during that period.
Tuchman highlights how the Crusades were seen as a means to unify and strengthen the Christian faith by addressing the internal divisions and external threats facing it. This historical perspective underscores the ideological motivations behind the military campaigns launched to reclaim the Holy Land and restore order within Christendom.