Christianity reached Cologne long before it had an archbishop. Saint Maternus, attested at a Rome council in 313 as a contemporary of Constantine, is the earliest historically certain bishop of the city, presiding over an ordinary diocese for nearly five centuries. What changed its rank was Charlemagne's reorganization of the church in his expanding empire. In 795 he elevated Cologne to an archdiocese and appointed his own chaplain, Hildebold, who was already serving as its bishop, as the new archbishopric's first archbishop.
The promotion was not merely honorary. Hildebold received metropolitan authority over a cluster of suffragan sees across the Lower Rhine and further north, including Tongres-Liege and Utrecht, making Cologne the senior church of a genuinely large ecclesiastical province rather than a single city's bishopric. That structure proved durable. Cologne remained a metropolitan see governing multiple dioceses for the rest of the medieval period and into the modern one, and the office Hildebold first held grew, by the mid-13th century, into something considerably larger than a spiritual post: its holders became prince-electors of the Holy Roman Empire, ruling a temporal territory, the Electorate of Cologne, alongside their religious authority, a dual role that lasted for roughly five and a half centuries after Hildebold's own appointment before Napoleon-era reforms finally separated the two.