Turpin is Charlemagne's Archbishop of Rheims but also a pretty sweet knight, who embodies the tradition of the church militant or fighting Christianity. History time: a century before the Oxford Manuscript was written, Pope Urban II got the Crusades started with a speech urging faithful European Christians to re-conquer the Holy Land.
In the 12th century the idea of fighting for God was still going strong. Most religious people, especially the cloistered ones like nuns and monks, tried to stay out of medieval campaigns, but Archbishop Turpin represents the exceptions: clergymen who love God so fiercely they aren't content to stay holed up in cathedrals. They want to fight, dangit! As he says to Roland, praying is all very well for monks who can't fight, but for strapping men like Roland and himself? They serve God with swords (141).
And Turpin does, although you'll notice that he also maximizes his religious training for the good of the other fighting Franks. With Oliver he preaches inspiring things to the knights to keep their spirits up, but also does the additional work of promising them a martyr's salvation and absolving them of their sins.
"We must die well for our King:
Help sustain Christianity! […]
I will absolve you to save your souls.
If you die, you'll be holy martyrs,
You'll have seats in highest Paradise." (89.1128-29, 1133-35)
He validates what every other knight already believes: that religion and war are natural partners. What's the best way of sustaining Christianity? By conquering non-Christian lands, pillaging their infidel places of worship, and forcing their people to get baptized. Frankish Christianity is definitely not a religion of peace, but a super-buff, absolutist, and violent way of believing.