There is a common misconception that the knights that chose to travel to the Holy Land for the First Crusade were minor nobles from cadet branches, interested in potential personal gain. The many written accounts of the crusade (the best documented crusade and one of the best documented events of the middle ages) don't give that impression at all. People going on that crusade truly felt that they were soldiers fighting in Christ's army. The knightly ethos that would compel them to protect their king if his land were to be taken unjustly, fit perfectly with their view of Christ-the-King wronged by the Saracens, who had taken what was rightfully his, the holy city of Jerusalem.

If anything, we have written sources that show that knights and their families were investing a lot of personal funds on this armed pilgrimage, with no prospect for financial gain. The cost of funding one knight for the long and arduous journey was staggering, often more than three times the annual income of the family of a landed knight, and there are many written records of land and properties sold or mortgaged to raise funds for the crusade. And of those who survived the journey and made it to Jerusalem, nearly all returned home, bringing back no significant riches, mostly just relics of the Holy Land.

Among the elite leaders of the crusade, honest religious fervor motivated them, according to the many accounts of the crusade. Even Bohemond of Taranto (a Norman from southern Italy, the son of Norman knights who did travel from France to Italy around the middle of the eleventh century for the potential of loot and land, and ended up conquering all of southern Italy and Sicily), is not singled out in the chronicles as one interested in personal gain, even though he ends up becoming the ruler of the Principality of Antioch. And we can't even blame the written sources as being biased in favor of the knightly class, since nearly all of the chronicles of the crusade were written by clergy, and tend to narrate the events from the point of view of the many religious men who went along on the crusade.

But Jerusalem, together with the major episcopal sees of Antioch and Alexandria, had been conquered by the Arabs in the seventh century, about 450 years before the call to a crusade by Urban II. So why was there suddenly a great urgency to liberate the holy land at the end of the eleventh century, when it did not seem to be a significant priority for the previous several centuries?

The level of religious fervor demonstrated by the nights and by "commoners" who went on the first crusade was fairly new. One significant factor was the presence of millenarist ideas, with many people believing that the end of the world was near. In 1064-65, about 7000 people went together on a pilgrimage from Germany to Jerusalem, believing in the millenarian significance of the date of Easter for that year, that they thought to be the same date in which Christ was resurrected. This strain of lay Christian faith is at the forefront of a movement that will span several generations in a variety of forms, and in the thirteenth century can be found on one hand in the Cathars in Italy and France, strongly opposed by the church as heretics, and on the other in the mendicant orders, Franciscans and Dominicans, which were instead accepted by the church. The accounts of the first crusade include many charismatic religious figures, visionaries and mystics, such as Peter the Hermit and Peter Bartholomew, who play an important role in the crusade, especially when the knights and the rulers hesitate in advancing towards Jerusalem.

The church of the late eleventh century is also quite different from the one of the time of the Arab conquest of the holy land. The papacy had been asserting its primacy over the temporal leadership, and had been defining Christianity in ways that were different from the Christianity of the early middle ages. The emphasis on celibacy of the clergy is an example of this "new" Christian church, along with the whole investiture controversy and the great schism. The papacy starts to redefine what it means to be Christian, a process that will continue through the Lateran Council of 1215 and beyond. One important way to define what meant to be Christian was to define what was not Christian. Theological ideas that were accepted and normal a century before became heresies. Jews, who were tolerated and in mostly in good terms with the Christian communities in which they lived, were seen as the people of the Old Testament prophets, but now become the "other", leading to a long season of persecution throughout Europe. Arabs are also "others", becoming enemies after being relatively peaceful neighbors in Sicily and the Iberian Peninsula for centuries. The language used in the call to the crusade by Urban II at the Council of Clermont depicts the Arabs in very violent terms, describing the presumed crimes committed against Christians in the Holy Land and the defilement of the holy sites. They are "others" and enemies of Christianity. And Urban II talks about the loss of Jerusalem to the Arabs as if it just happened, not as an event from past centuries. Everything is urgent, and violent.

The world of the late eleventh century was in many ways different from the world of the Arab conquest, from the world of the Carolingian empire and the one of the Ottonian dynasty. Western Europe had just entered a period of dramatic transformation in culture, economy, demography, and especially in the Christian religion. The crusading ideal suddenly had fertile ground to take roots and grow.

Sources:

Kostik, Conor. The social structure of the First Crusade. Brill.

Moore, Robert I. The formation of a persecuting society: authority and deviance in Western Europe, 950-1250. Malden, Mass.: Blackwell Publishing, 2009.

Riley-Smith, Jonathan. The first crusade and the idea of crusading. London: Continuum, 2012.

Wickham, Chris. Medieval Europe. New Haven: Yale university press, 2017.