Christopher Tyerman outlines the passages favoured by Crusaders to justify their cause.
The propaganda around the Crusades was heavily based on various texts in the Bible. So you have the Old Testament texts – the violence with Joshua; the Maccabees; the idea that God instructs his followers to kill people. He instructs the Levites to kill the worshipers of the golden calf; he instructed Saul to destroy the Amalekites, men, women, children, animals, the lot, and when he doesn’t commit full genocide, God withdraws his favour from Saul. There are the Maccabees who fight for Jewish independence in the 2nd century BC. Again, Judas Maccabeus is a figure that is reworked for crusading purposes.
There’s also, of course, the Book of Revelation, the idea that the apocalypse will take place with the violent extermination of the forces of the antichrist. And there’s this vision in the Book of Revelation, of Christ himself descending from heaven with a sword in his teeth to destroy his enemies. And the scene of that apocalypse is obviously Jerusalem. So there are these very strong texts.
There is, of course, the other text, of Christ repeatedly saying in the New Testament, “Take up your cross and follow me”. And that is used again and again as a leitmotif of crusade propaganda.