At the time of the Crusades, the official church had become corrupt and politically motivated. It should be noted, too, that crusaders did not take vows to “go on crusade.” The very term crusade, in English or in any other language, is a much later invention. What we call “crusades,” contemporaries knew as “pilgrimages” or even simply “journeys.” Aside from a tiny elite, people were illiterate and even if they could read, there was no access to a Bible or any scriptural teaching. It was an age of superstition and magic, where visions, signs and wonders were claimed by many. The masses’ only source of knowledge about God was whatever the often corrupt and greedy clergy decided to teach. The early crusades broke new grounds for the power of the Church. It gave the Pope the highest order of command and brought about new religious vows.
The crusade was a holy war, which differed from earlier wars against the enemies of Christendom in that it was waged by command of the pope. In order to ignite the fire of the Crusades, the Pope had to take existing theology about violence and warfare and turn it on its head.
Until this time a Christian soldier had to do penance for any violence in order to reduce the time he would spend in purgatory before going to heaven. Violence was considered a necessary evil, but nevertheless still evil. Popes Gregory VII and Urban II changed that. They said that an act of warfare against the infidel, i.e. the Muslims, was in itself an act of penance, and if a Christian were to lose his life so doing, he would go straight to heaven. Many Crusaders extended this concept to include killing Jews too. With many people facing what they believed could be countless years in purgatory, it is no wonder that tens of thousands willingly gave themselves to the cause of killing the “infidel”.
Regardless of motivation, an individual underwent a specific ceremony before he could be considered a “crusader.” The ceremony evolved somewhat over the centuries, but its general outlines remained the same. A would-be crusader sought out an ecclesiastical authority (a priest, bishop or higher cleric) and swore to carry out an armed “pilgrimage” in support of the Holy Places. He then usually received a cloth cross which he could place on his clothes to signify his new status.
Crusading vows were usually taken in response to official preaching of a crusade by licensed churchmen. They were supposed to be taken only by fighting men or those who could otherwise contribute to a military effort, and they were not to be taken without the permission of the crusader’s wife, since his long absence would deprive her of what was delicately called “marital rights” (Pope Innocent III, in need of troops for his crusading proposals, changed this in the thirteenth century, but in doing so he violated longstanding Church tradition and the plain intentions of canon law).
The crusader’s property and people were then placed under the protection of the Church, and he was to begin preparing to leave. If he did not discharge his vow within a certain period of time, he might be excommunicated by the church until he kept his word.
Crusaders were often offered an indulgence in return for participation in the hardships of a crusade. The indulgence was later seriously abused, and the word acquired a justifiably obnoxious connotation. But in the beginning it was another of those carefully thought out doctrinal innovations that attended the reforms of the tenth and eleventh centuries.
In brief, the indulgence assumed that if an individual were truly penitent for his sins, he might obtain remission or forgiveness for the temporal penalties of those sins by performing some arduous, virtuous or unpleasant task to compensate for them. This remission could apply to penalties imposed by the Church on earth (i.e., to penance prescribed for sin), and it might also apply to penalties imposed by the Church in the next world (i.e., to time spent in purgatory).
Most medieval people were deeply interested in their fate in the next world, and the indulgence was a powerful incentive to participate in crusades. It was especially effective amongst the very people whom the Church was trying to recruit: the baron who was a competent warrior but who had perhaps been applying that competence to unlawful targets such as other Christians and who, as a result, had a guilty conscience.