A vault adorned with mosaics provides light for the apse, the floor and walls of which are faced with marble. These are the verses which describe the scene depicted on the vault:

The Trinity shines out in all its mystery.  Christ is represented by a lamb, the Father’s voice thunders forth from the sky, and the Holy Spirit flows down in the form of a dove.  A wreath’s gleaming circle surrounds the cross, and around this circle the apostles form a ring, represented by a chorus of doves.  The holy unity of the Trinity merges in Christ but the Trinity has its threefold symbolism.  The Father’s voice and the Spirit show forth God, the cross and the lamb proclaim the holy victim.  The purple and the palm point to kingship and to triumph.  Christ Himself, the Rock, stands on the rock of the Church, and from this rock four plashing fountains flow, the evangelists, the living streams of Christ.