AHC - Medieval Europe ruled by a theocracy

A big theme in Medieval European history was the relationship between the Pope and secular rulers. The Pope was indeed a powerful figure, but in the end, the Papacy was never able to wield total control in Western Europe while kings were able to grow in strength and authority.

My challenge to you, with a PoD somewhere around the time of Constantine, is to create a system where most if not all of Western Europe by the High Middle Ages is ruled by a Pope-Emperor who wields complete temporal and ecclesiastical authority.

I think it would be necessary to go all the way back to the ecumenical councils to prevent widespread agreement among the bishops. Thus, when the Arian Germans take over, they are not a religious minority ruling over an established church, but are able to lead the theological debates in a manner that causes them to be recognized as heads of the church. Thus, certain kings (such as, but not limited to, the Franks) create a tradition of being recognized as apostles of Christ, and are therefore given control over the church as well as the state. Any thoughts?
 
Western Rome survives and stays united barely and only loses the British Isles at most. It still becomes fully Christian. The emperor becomes a puppet of the pope who holds a special religious position within the empire. The rest of the empire is ruled by church officials, senate members, and governors at the local level.
 
Kinda uncertain if the best way to go at it would be to wank the Papal state (making him much more able to control the secular rulers), or to go into the direct opposite direction, screwing the Papal state over as to a point where there aren't any single recognized head of church but rather the ability for every ruler to declare themselves a "Christian Caliphate", which in turn leans much more on the ecclesiastical class for support, and even some places being drawn from them instead of a more feudal process ... This would probably mean that Christianity would mean slightly different things in Spain, than in Britain, than in Italy, where every nation could either be defined as either being an Autoephaly (optionally with an even more powerful head of chruch, depending on exact location), or outright different denominations which might lean towards whichever religious thought that is popular/powerful in the area
 
Western Rome survives and stays united barely and only loses the British Isles at most. It still becomes fully Christian. The emperor becomes a puppet of the pope who holds a special religious position within the empire. The rest of the empire is ruled by church officials, senate members, and governors at the local level.
A transition of power from Augustus to Pontifex Maximus ?
 
The Kaiser decides to put the Papacy in its place during the upsets just before the Great Schism, reducing the relationship between Pope and Kaiser to something like that between the Patriarch and Basileus?

Later, as France and England grow, the King of France appoints a new Antipope in a similar role, and a similar relationship develops between the King of England and Archbishop of Canterbury.
 
A transition of power from Augustus to Pontifex Maximus ?
Maybe, I was thinking a similar relationship between emperor and pope as the one between emperor and shogun in Japan forms in the western Roman Empire. The emperor only rules in name but the pope holds the real power. He also the one that crowns the emperor and appoints people to power in both religious and secular matters. Kind of the opposite to the eastern Roman Empire where the emperor is the highest power. When Christianity spreads to Germany and Central Europe something similar to the Holy Roman Empire(different name and title of course but same political structure)still forms where the pope holds similar influence and power to the one in our world there. If that confederation of states acts against his wishes or demands he uses the western Roman Empire to enforce himself there. In other Catholic nations he still holds similar influence and power to our world but with the backing of Rome he can more easily make Catholic nations and leaders bend to his will.
 
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