United Europe as a papal theocracy

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?
 
Maybe entirety of Europe can come under the HRE and well, be HRE-ised, complete with a near-powerless elective monarchy-- But in this case, HRE is actually considered a state, because the individual nobles do not fight each other, but has the pope settle disputes, and there is an "Pan-European Army" of sorts-- a more modern, more unified version of the Crusader Armies, with the Emperor as the commander. The relation between the Emperor and the pope, could be like that between a modern ruler, and his commander-in-chief, with the pope being able to force a new coronation if the Emperor goes out of line. A really long war (~300 years) with a non-christian empire shortly after the Crusades and relative peace between christian nations in that time period might do the trick... with wars not being allowed between Christian states might become a part of formal Catholic theology, making the Christians permanently allied to each other.

Maybe the Mongol Invasion of Europe will create this Holy League on steroids?

Not a far stretch from OTL considering the Crusades and the power of the pope even into Early Modern Times, we just need to stifle nationalism somehow and make Catholic theology more socially and politically engaged
 
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