Christopher Tyerman explains how medieval warrior culture influenced Christianity in Europe.
After the fall of the Roman Empire, western Europe was dominated by military aristocrats whose whole cultural and social justification was their use of violence to promote their own power, to protect their subjects, to exploit natural resources. This was how aristocratic society operated – the culture of the warrior: the warrior band; the virtues of bravery, loyalty, honour; the mutual gift-giving between warriors. And this of course caused a problem for the church.
The church, however, accommodated this. That you see in early medieval texts popularising Christianity – that Christ and his apostles, Christ becomes a knight, the apostles become his war band. And he defeats the devil when he goes down to hell after the crucifixion, before the resurrection. This is seen in terms of a battle. Visions of heaven are in a sense rather like Valhalla.
So the way in which Christianity accommodates the dominant political and social culture of the warrior class is absolutely central to the way in which religious violence becomes an accepted norm.