Is it possible to have all of Europe united, more or less under the Pope? I mean, it doesn't have to be centralized-- with a chain of vassals, more or less domestically independent, and it might not explicitly be called a "papal empire/theocracy", but whatever leader/emperor Europe has is a lot less powerful than the pope, and essentially a subject to him. And the populace, down to the peasantry are much respectful of the pope than their temporal rulers,with "The Donation of Constantine" myth being accepted as fact, at least in popular imagination.
Any ideas on how this might be accomplished?