To Cut a Long Story Short
The history and development of what we blithely like to call "Christianity" is murky.
By the time of Emperor Justinian (527 - 565) the Roman Empire had long upped sticks and moved East to Constantinople (now Istanbul). Justinian set about recreating the old glory days, re-conquering lost territory and codifying Roman law. This put the Bishop of Rome aka the Pope in a panic because the Latin Christians had by that time developed such a megalomaniac power complex that, in their own minds the Pope has automatic entitlement, as Head of the Church of Rome to be Supreme Ruler of the World. Unfortunately for them the Emperor thought different. As did the Patriarchs and Greek Christians of the East.
So the Popes and the Latin church dreamt of a Cunning Plan. They needed to develop an alternative power base, outside the control of the Emperor and the Greek Christians, so the Popes looked West, and had the idea of turning the barbarian peoples of Iberia, Gaul and England to their purpose. The Roman church had already forged documents claiming that St Peter had bequeathed the "binding and loosing" powers that he had received from Jesus (Matthew 16) to the Popes in perpetuity, and despite making a muddle of supposedly giving the powers to Clement when it is otherwise recorded that Linus was the first Pope after Peter, they got away with it.
To give themselves maximum authority another crucial document was forged. Calling it 'the Donation of Constantine', the Popes claimed that Emperor Constantine (c. 272 - 337) had become a Christian and simply given away his all-important powers, both regal and sacerdotal, to the Bishop of Rome. Just like that.
Armed with these spurious powers that they had either faked or usurped or both, the Papacy was in contact with Pippin, a Frankish official under the Merovingian kings who himself wished to usurp the kingship from his master. In a very handy arrangement the Papacy backed Pippin to be king, while Pippin in turn subordinated himself and gave his fealty and support to the Papacy. Under Pippin's son, Charlemagne and his successors, the relationship between the Frankish kings and the Popes became ever deeper, each bolstering the other in their expanding power.
This process necessarily involved also the firm inversion of the Germanic tribes' native governance systems in which power came from the people and was temporarily given to leaders of the people's choice and where the people could remove leaders who weren't doing what people wanted them to do.
Instead, under the Latin Christian system, power comes only from God [sic] and it is passed directly from 'him' to the Pope, who then 'graciously' crowns kings as he likes, Deo gratias. The king in turn passes power down through a hierarchy, as conveniently expounded by Pseudo-Denys. People under this system of Theocracy were thus forbidden from any say in governance at all.
Under the Frankish ecclesiastical hierarchy the forgeries continued apace, now aimed at buttressing "the hierocratic thesis by surrounding it with the halo of antiquity".
More in "Medieval Political Thought", Walter Ullmann (Peregrine 1975).
This was only the early stages. Once this system had been set up, the Papacy in alliance with the newly Christianised Barbarians and Viking Pirates such as their strong-arm enforcers, the Normans, could move forward, terrorising and subjecting people in the West to their domination, before then expanding their Rule of Terror further to attack the Byzantines and the Middle East ...