Complex sentence: The Episcopal see was established in ancient Roman times in the city of Mainz, which had been a Roman provincial capital, Moguntiacum, but the office really came to prominence upon its elevation to an archdiocese in 780-782.
Split into simpler sentences: The episcopal see was established in ancient Roman times in the city of Mainz, which had been a Roman provincial capital, Moguntiacum. However, the office really came to prominence upon its elevation to an archdiocese in 780-782.