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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?
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