St. John’s Church (Johanniskirche), Mainz

Saint John’s Church (Johanniskirche) in Mainz is a remarkable historical site located beneath the renowned Mainz Cathedral. This 1100-year-old structure stands as the oldest church in Mainz and holds the distinction of being the oldest preserved cathedral in modern-day Germany. Originally serving as the first episcopal seat for the Bishop of Mainz, it is an extraordinary example of Carolingian and early Ottonian architecture.

The church was first constructed in the 7th century, with its main structure built during the early medieval period. Its design is predominantly Carolingian, though various architectural additions over the centuries have introduced a mix of styles. The building features three naves and is dedicated to John the Baptist.

The church has been intricately connected to the history of Mainz. For a time, it served as the cathedral of the Bishop of Mainz, until the appointment of Archbishop Willigis in 975. Notably, Archbishop Erkanbald, who was buried in the church in 1021, is one of the prominent historical figures associated with Saint John’s. Recent archaeological discoveries in 2019, including a sarcophagus, have shed light on the early history of the diocese, although ongoing investigations continue to determine whether the remains belong to Erkanbald.

Saint John's architecture reflects its long history, with the original building from the 7th century built over an earlier Roman or Christian structure. Several reconstructions occurred, including a key renovation by Archbishop Hatto I. His version, known as Hatto’s Church, includes a double chancel design, arched and "Oeil-de-boeuf" windows, and a clerestory with closed-off arched windows above arcades.

Archaeological studies reveal seven-floor levels and 6th-7th century masonry at Saint John’s Church, pointing to its early Christian roots in Mainz, possibly on Roman foundations. Despite changes, it remains a key link to the city's Carolingian and Christian history.