Tool theory, bro, but it's not true. First, most of the people who responded to the call to crusade weren't knights at all. They were poor people. And secondly, most of the nobles who did go crusading were lords of estates, not their wastrel kids. But more importantly, that analysis ignores religious motivations. We've approached religions as historical phenomena, thinking about how, for instance, the capricious environment of Mesopotamia led to a capricious cadre of Mesopotamian gods. But just as the world shapes religion, religion also shapes the world.