Of the abbey church consecrated by John XIX in 1024, little can be seen in the interior except the mosaics in the narthex and over the triumphal arch, the medieval structures having been covered or destroyed during the "restorations" of various abbots in commendam. Some fragmentary thirteenth-century frescoes were revealed in a partial restoration of the church in 1904 to commemorate its novecentennial, when it was made a Roman basilica. The mosaics portray the Twelve Apostles sitting beside an empty throne, evoking Christ's ascent to Heaven.