The pope, alongside the Holy Roman Emperor, was one of the universal powers of the European Middle Ages. The Roman bishops regarded themselves as successors of the apostles Peter and Paul, who, according to tradition, were active in Rome and gained continuous influence from Late Antiquity until the High Middle Ages.
This was, however, no straight-line process, but was interspersed with numerous influential popes on the one hand and darker phases on the other hand. While we know very little about the earliest popes and have to rely on chronicles, especially the Liber pontificalis, the papacy increasingly shaped its profile from the 4th century onward, due to the end of the persecution of Christians and the Donation of Constantine. After the imperial residence had moved from Rome to Constantinople, the Roman bishop took over crucial functions concerning the administration, protection (Leonine Wall), and supply of the city. Nevertheless, the following centuries were influenced by an often tensed relationship to the East Roman emperor and the patriarch of Constantinople. Rome was not able to enforce its claims of supremacy over the other patriarchates (Alexandria, Antioch, Constantinople and Jerusalem) in the era of the great synods; in fact, the Roman church played a subordinate role in theological discussions about the nature of Christ. A prominent pontificate in these times was the one of Gregory the Great (590-604), and it is not by chance that a register of his letters has been preserved.
The popes' self-conception was also changing continuously over the course of the centuries and eventually culminated in Pope Gregory VII's famous Dictatus papae from 1075. This widely discussed document, written in the context of the Investiture Controversy, expresses the pope's claims to power. It is one of the most important documents of the time of the reforming popes. From this time on, there was an effort to return to the popes' clerical duties, distancing themselves from the previous "dark century" of papal history, in which the papacy had become a plaything of local noble families.
However, the involvement with the secular sphere did not come to an end with the reforming popes. Not only was their claim of supremacy in spiritual matters generally accepted in the Western Christian world, the popes also greatly influenced the crusades. Even after the so-called Investiture Controversy came to an end with the Concordat of Worms in 1122, conflicts with the King of the Romans arose repeatedly, especially between Pope Alexander III and Emperor Frederick I.
The Regesta Pontificum Romanorum contribute greatly to a reevaluation of the papacy's role in contact with various persons and institutions. The lower levels of ecclesiastical and secular hierarchies are treated equally, including the so-called peripheries, such as the Iberian Peninsula and the Eastern church provinces. This results in a more detailed framing of papal influence of Europe's homogenisation.