The church can be visited
St.-Johannis-Church

St. John's Church in Mainz is the oldest church in the city and the only preserved cathedral structure from the late Carolingian and early Ottonian periods in Germany. It was originally used as the cathedral of the Archdiocese of Mainz.
History+
Archbishop Hatto I began the reconstruction of the then-cathedral church around 900, which was consecrated in 910. Until the consecration of the new Mainz Cathedral in 1036, St. John's Church served as the cathedral of the Archdiocese of Mainz and was dedicated to Saint Martin of Tours. After the completion of the new cathedral, it became known as “Aldedum” or Old Cathedral and housed a collegiate church. In 1002, King Henry was crowned in this Old Cathedral.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Johanniskirche_mainz.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 49.9984, 8.2724