The mosaics in the apse of the church of San Marcowere added during the reign of Pope Gregory IV (r. 827-44).
In the centre of the apse stands Christ, holding an open book in his left hand. It reads: EGO SVM LVX EGO SVM VITA EGO SVM RESVRRECTIO (I am the light. I am the life. I am the resurrection).
To the left stand Pope Gregory IV (holding a model of the church), St Mark and St Felicissimus, while to the right stand St Agnes (holding a martyr’s crown), St Agapetus and Pope St Mark (r. 336). The hand of God the Father can be seen above the head of Christ, while under his feet we see a dove with a halo, perching on the edge of a fountain.
In the centre of the lower band stands the Agnus Dei (Lamb of God) flanked by twelve sheep (symbolising the twelve apostles). At each end of the sheep are the walled cities of Bethlehem and Jerusalem.
On the outside of the arch are roundels of Christ and symbols of the four Evangelists. The two full-length figures depict St Paul and St Peter.