Glendower: I can call spirits from the vasty deep.
Hotspur: Why, so can I, or so can any man; But will they come when you do call for them? (Shakespeare: HENRY IV pt I)
I don't believe it was simply a case of the "Church" calling for a crusade. There were all sorts of economic, social, intellectual, and God knows what else factors, pointing in this direction. It was the old business, "I am their leader, I must follow them."