Obviously I didn't give the whole story, that would take a PhD and about 400 pages. No one wants to read what I have to read for that long. And of course, there were other motivations for the Crusaders, there always are. Many of them were not their for the Crusade, but for purely mercenary reasons. I am not familiar with Mr. Johnson, what else would he contend motivated the Crusades?
As a substitute for penance and source of indulgences (page 233): "In 1095, Urban II, propagating the first crusade, laid it down that a crusade to the Holy Land was a substitute for any other penance and entailed complete remission of sin....Throughout the twelfth century, crusading was the only source of indulgences..."
Migration (page 244): "What really created the crusade...sprang from the vast increase in western population in the eleventh and twelfth centuries, and the consequent land hunger...The idea that Europe was a Christian entity, which had acquired certain inherent rights over the rest of the world by virtue of its faith, and its duty to spread it, married perfectly with the need to find some outlet both for its addiction to violence and its surplus population."
Racial arrogance (page 45): "From the start, then, the crusades were marked by depredations and violence which were as much racial as religious in origin."
Ecclesiastical control (page 249): "It is, in fact, a misleading over-simplification to see the crusade simply as a confrontation between Christian Europe and the Moslem East. The central problem of the institutional church was always how to control the manifestations of religious enthusiasm, and divert them into orthodox and constructive channels. The problem was enormously intensified when large numbers of people were involved...A crusade was in essence nothing more than a mob of armed and fanatical Christians." (page 250): "Naturally, when antinomian mobs were liable to sweep away church institutions, established authority was anxious to get them out of Christendom--preferably in the East, whence few would return."
Paul Johnson, A History of Christianity, A Touchstone Book/Simon & Schuster, 1976; first Touchstone edition 1995.