@liaizon I think the best candidate would be some Roman Catholic diocesis. Regarding German dioceses, there is an account for @bistumtrier, which dates back to Roman antiquity, although I cannot decide whether it is an official account. @bistumpassau, @bistumwuerzburg, @BistumAugsburg were all founded the 8th century. There might me many accounts for very old dioceses in other countries as well.

Roman Catholic Diocese of Trier dates back to 250AD ( @bistumtrier)

The Roman Catholic Diocese of Würzburg was founded in 741 or early 742AD ( @bistumwuerzburg)

The Roman Catholic Diocese of Augsburg dates back to possibly the 11th century but seems the exact date is uncertain ( @BistumAugsburg)

Thanks for discovering these accounts @steffenloewe!