You've got to be pretty creative to get anything like "holy war" out of the New Testament, what with all that trespass-forgiving, cheek-turning, and neighbor-loving. By all appearances Jesus didn't want his followers to fight for their faith, but rather to die for it as he had. And during the first three centuries of Christianity–in the time of the Roman persecution–that's just what they did. "To die in Christ is to live," wrote the Apostle Paul. And it seems a lot of early Christians believed him for they sought martyrdom. Jesus passively gave his life; and they passively gave theirs. What could be more fitting?
All this passivity makes the Crusades seem very strange indeed. If Christ's message was one of peace, what in the world were Christians doing taking up arms in the his name? In his excellent Armies of Heaven: The First Crusade and the Quest for Apocalypse (Basic Books, 2011), Jay Rubenstein explains that the reason they did so had everything to do with the conviction that the world was going to presently end. The Crusaders fervently believed that the closing chapter in temporal history upon them and that they had a role in bringing it to the right conclusion. They didn't know exactly what that role was, but there were good hints in ancient scripture and contemporary signs. Everyone agreed that, whatever part the Crusaders were to play, it involved liberating Jerusalem from the infidels. So off they went. Since they were on a holy mission–in fact the last holy mission before Christ's return–the ordinary rules did not apply. The Crusaders forced Jews to convert or else die (many were murdered). They killed Muslims indescriminately. They made sport of desecrating the bodies of their victems. They even roasted some on spits and ate them. That's right: they roasted and ate them. It was like something out of the Book of Revelations. Which made sense, because the Crusaders believed they were in the Book of Revelations.