The Crusades may be regarded partly as the decumanus fluctus in the surge of religious revival, which had begun in western Europe during the 10th and had mounted high during the 11th century*. . . . Considered as holy wars the Crusades must be interpreted by the ideas of an age which was dominated by the spirit of otherworldliness, and accordingly ruled by the clerical power which represented the other world. They are a novum salutis genus – a new path to Heaven, to tread which counted "for full and complete satisfaction" pro onni poenitentia and gave "forgiveness of sins." . . .; they are, again, the "foreign policy" of the papacy, directing its faithful subjects to the great war of Christianity against the infidel. As such a novum salutis genus, the Crusades connect themselves with the history of the penitentiary system; as the foreign policy of the church they belong to that clerical purification and direction of feudal society and its instincts, which appears in the institution of "God's Truce" and in chivalry itself. The penitentiary system, according to which the priest enforced a code of moral law in the confessional by the sanction of penance –penance which must be performed as a condition of admission to the sacrament of the Eucharist – had been formerly times a great instrument in the civilization of the raw Germanic races. Penance might consist in fasting; it might consist in flagellation [whipping]; it might consist in pilgrimage. The penitentiary pilgrimage, which seems to have been practised as early as A.D. 700, was twice blessed; not only was it an act of atonement in  itself, like fasting and flagellation; it also gained for the pilgrim the merit of having stood on holy ground. [Pilgrimages became increasingly frequent.] . . .
   When the First Crusade finally came [1099], what was it but a penitentiary pilgrimage under arms – with the one additional object of conquering the goal of pilgrimage? That the Pilgrims' Progress should thus have turned into a Holy War is a fact readily explicable, when we turn to consider the attempts made by the Church, during the 11th century to purify, or at any rate to direct, the feudal instinct for private war . . . . [I]t was an easier thing to consecrate the fighting instinct than to curb it; and the institution of chivalry represents such a clerical consecration, for ideal ends and noble purposes, of the martial impulses which the Church had hitherto endeavoured to check. . . . As chivalry directed the layman to defend what was right, so the preaching of the Crusades directed him to attack what was wrong – the possession by "infidels" of the Sepulcure of Christ. . . . . The knight who joined the Crusades might thus still indulge the bellicose [combative/fighting] side of his genius – under the aegis [sponsorship] and at the bidding of the Church; and in so doing he would also attain what the spiritual side ardently sought – a perfect salvation and remission of sins. He might butcher all day, till he waded ankle-deep in blood, and then at nightfall kneel, sobbing for very joy, at the altar of the Sepulchre – for was he not red from the winepress of the Lord? One can readily understand the popularity of the Crusades, when one reflects that they permitted men to get to the other world by fighting hard on earth. . . . Nor was the Church merely able, through the Crusades, to direct the martial instincts of a feudal society; it was also able to pursue the object of its own immediate policy, and to attempt the universal diffusion of Christianity, even at the edge of the sword, over the whole of the known world.
   Thus was renewed, on a greater scale, that ancient feud of East and West. . . . From this point of view, the Crusades appear as . . . a reaction which carried the West into the East, and founded a Latin and Christian kingdom on the shores of Asia. They protected Europe from the new revival of Mahommedanism under the Turks; they gave it a time of rest in which the Western civilization of the middle ages developed. . . .
   In 1074 Gregory actually assembled a considerable army; but his disagreement with Robert Guiscard, followed by the outbreak of the war of the investitures, hindered the realization of his plans, and the only result was a precedent and a suggestion for the events of 1095. . . .
   The primary force, which thus transmuted an appeal for reinforcements [to protect Constantinople] into a holy war for the conquest of Palestine, was the Church. . . . But it would be a mistake to regard the Crusades . . . as a pure creation of the Church, or as merely due to the policy of a theocracy directing men to the holy war.  . . . It would be almost truer, though only half the truth, to say that the clergy gave the name of Cursade [from the word "cross"] to sanctify interests and ambitions which, while set on other ends than those of the Church, hapened to conside in their choice of means. There was, for instance, the ambition of the adventurer prince, the younger son, eager to carve a principality in the far East, of whom Bohemud is the type; there was the interest of Italian towns, anxious to acquire the products of the East more directly and cheaply, by erecting their own emporia in the eastern Mediterranean.
   * Note: The 10th and 11th centuries saw the peak of the power of the Roman Catholic church which arose to significance in the 6th century and reached its low point in the 18th.