I think a cursory glance at the quotation you included here would lead one to conclude that Charles Oman had a spectacularly romantic view of medieval history, and that his representations of Hungarian, Scandinavian, and non-Christian peoples betrays the very typical biases one can encounter in early twentieth century British historians.

Your introduction into his work began by discussing military campaigns in the ninth and tenth centuries as examples supporting Oman's thesis that mounted warriors were essential in this early "heroic" struggle against the forces of paganism and so forth. The problem is that if we consider the ninth century, cavalry tactics differed substantially from the kinds of maneuvers employed in the eleventh and twelfth centuries. Bernard Bachrach has gone so far as to state that not a single significant battle or campaign has been cited in which cavalry can be shown to have played the tactically decisive role." Bernard Bachrach, "Charlemagne's Cavalry: Myth and Reality" Military Affairs 47 (1983), 181.

John France has also contended that it is by no means certain that cavalry played the central decisive role in Carolingian warfare that earlier scholars had assumed, and that their responsibilities were more commonly directed towards raiding and reconnaissance. Indeed, France has commented on Oman's thesis directly in his assessment of the Battle of Hastings where he wrote that the battle was not a classic example of the superiority of cavalry over infantry but rather good tactical command. Norman cavalry did not advance at speed in formation with "lances couched" but rather feigned and darted around the battle space harassing and selectively engaging men as they found them. John France, Victory in the East: A Military History of the First Crusade (Cambridge: Cambridge University Press, 1994), 71-72.

Now naturally other scholars have disputed Bachrach and France's characterization of the significance of cavalry in the ninth century but their work should nonetheless substantially complicate Oman's view and give you some pause.

Secondly the development of the knight along with conceptions of "knighthood" and "chivalry" was a very slow and gradual process that was still in its infancy in the late eleventh century. Far from being the defenders of Christendom glorified by Oman it is instructive to reflect upon the famous comments of Leo IX who described French knights in Italy as acting "with an ungodliness worse than that of the pagans." Richard W. Kaeuper, *Holy Warriors: The Relgiious Ideology of Chivalry" (Philadelphia: University of Pennsylvania Press, 2009), 12. Kaeuper cites a contemporary biography in "The Life of Leo IX," printed in Robinson, The Papal Reform of the Eleventh Century, 149.

Moreover, while the "Christianization" or "sacralization" of military violence had indeed occurred at various points in time, including with reference to early medieval wars during the Carolingian and early post-Carolingian period against non-Christian pagan peoples this was by no means a universal view. By the eleventh century the killing of fellow human beings was a subject of vigorous discussion, particularly given the endemic state of seigneurial warfare throughout France and large regions of the Latin Christian West. Pope Gregory VII himself stated that knighthood was such a problematic profession that it "can scarcely be performed without sin." H.E.J Cowdrey, "Pope Gregory VII and the Bearing of Arms" in Montjoie: Studies in Crusade History in Honour of H. E. Mayer, ed. B. Kedar, J. Riley-Smith, R. Hiestand (Aldershot, 1997), 23. Knighthood was therefore a very controversial trade or vocation that brought with it an array of dangerous spiritual consequences. As an institution it only came to be rehabilitated in the minds of clergymen and wider Christian society through a complex process worthy of multiple monographs that stretched into the twelfth and thirteenth centuries (and beyond).

Certainly one of the most important steps in that process is the effort by reformers in the eleventh century to constrain knightly warfare and direct the martial energy of Christendom to the wider non-Christian periphery. The birth of the Crusades and armed penitential pilgrimage at the close of the century can be seen as a significant move towards the remoulding if you will of the purpose and function of knighthood. While still once again in its infancy, reformers and religious authorities looked towards the proto-chivalric warrior cultures of Western Europe and attempted to push them to embrace a set of values that was slowly becoming more coherently defined to promote public order and establish some sense of control. It was possible for knights to play a valuable function in the defence of Christendom, but there very existence nonetheless regularly imperilled the lives and property of communities across the breadth of Europe.

I would recommend, among others, the writings of Bernard of Clairvaux and Gratian who composed some interesting work on the legitimacy of bloodletting and the idea of Just War. I would also recommend the Cycle of William of Orange and the works of Chretien de Troyes which show how knighthood came to be depicted in the lay imagination in the twelfth century, and the kinds of values and idealistic principles that were beginning to be associated with the Christian miles.