Magnificent St Matthias Abbey is thought to be Germany's oldest Christian church. Begun in 1127, when the relics of the Apostle Matthias (Judas' successor) were found here, it was added to over the ensuing centuries. Treasures include the Holy Cross Chapel's Staurotheke (1240), containing a piece of Christ’s cross. Steps on either side of the Romanesque–Gothic interior lead to the crypt, home to the sarcophagi of the abbey's first two bishops, Eucharius and Valerius, and St Matthias' relics.