In the centre of the apse mosaic, flanked by a throng of angels, sits the Madonna and Child. The Pope, who sports a square halo (indicating he was still alive when the mosaic was executed), kneels by the Madonna's right foot, which he cradles in his hands. The ground is carpeted with flowers. In the intrados of the arch is the pope's monogram.
The long inscription proclaims: ISTA DOMVS PRIDEM FVERAT CONFRACTA RVINIS NVNC RVTILAT IVGITER VARIIS DECORATA METALLIS ET DEVS ECCE SVVS. SPLENDET CEU PHOEBVS IN ORBE QUI POST FVRVA FVGANS TETRAE VELAMINA NOCTIS. VIRGO MARIA TIBI PASCHALIS PREASVL HONESTVS CONDIDIT HANC AVLAM LAETVS PER SAECLA MANENDAM. (This house was once broken down in ruins, now it glistens for all time being decorated in various metals and behold God belongs to it. It shines like Phoebus in the world after the foul covering of horrible night flees. Virgin Mary, for you Paschal the trustworthy leader joyfully founds this house to remain for ever.)
The mosaic on the outside of the arch depicts Christ in a mandorla, an oval-shaped aureole. He is flanked by two angels, St Paul (left) and St Peter, and ten apostles. The two larger figures are thought by some to be St John the Baptist and St John the Evangelist. According to another school of thought, they represent Moses and Elijah.