Many of us have very little if any knowledge about the Crusades. We know they happened in the Middle Ages and involved knights. We know that it had something to do with the Middle East. But the why and who of the whole affair is distant and unimportant.
As a consequence of this, if your child or grandchild told you that the whole thing was caused by Christians it is very unlikely that you would be able to tell them why such a statement is incorrect. Who started the Crusades? Why did the Crusades start?