In taming the barbarians, the Church funneled their combativeness to the service of Christendom, giving rise to Chivalry and the Crusades
nobility.org
Providence disposed that the Middle Ages should be a specially warlike age. This was not only because of the double effort that Charlemagne had to do by fighting both Saracens and barbarians, but also because of the remaining traits of barbarian mentality in Catholics themselves, which led them to unsheathe their swords and combat for trivial motives, or for no reason at all.

The
Capitulation of Granada, Painting by Francisco Pradilla y Ortiz.
Boabdil gives the keys of the city to King Ferdinand and Queen Isabella
… Specifically, it was a twofold offensive against Christendom. The Church had to defend Herself, and She did so by stimulating this spirit within Her ranks. And from this came the encouragement provided for, and the sanctification of the Crusades.
(Excerpt from an Almoço Tuesday, Jan. 30, 1990 – Nobility.org)
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