Pre-11th century Christianity was rather peaceful, at least the version of it practiced by the masses (arguably the aristocratic version was not). But starting around the time of the Crusades (the Norman Conquest is another legitimate example), sometime in the mid-11th century, the popular Christianity of the peasants began to acquire a militant bent and commoner populations could be mobilized for war through religious appeals. Prior to this point, Christianity among commoner populations was a strange beast, a weird mix of pagan customs, folklore and Catholicism. The church during this time had a very difficult time suppressing pagan celebrations, enforcing orthodoxy, preventing these populations from worshipping Saints as patron deities, and so on, and had a difficult enough time simply keeping the religious practices and beliefs of the peasantry in line with that of the church... So, the militant version of Christianity which appeared in the mid-11th century was a drastic alteration of the way in which the Church related to the common classes and the kind of messages it sent them.