The mosaic in the apse dates from the 6th century and may be divided thematically into two sections:
The upper half illustrates the Transfiguration of Jesus Christ, steered by the hand of God. In this event, Jesus is revealed as divine to his disciples Peter, John, and James (here symbolized by the three lambs) in the presence of the two Old Testament prophets Moses and Elijah. The face of Christ is on the bejewelled cross at the center of a blue night with 99 silver and golden stars.
At the center of the lower level is Saint Apollinaris โ a position traditionally reserved for depictions of Christ. He is surrounded by twelve lambs symbolizing the disciples, or faithful in general. The mosaic is enhanced with a variety of trees, flowers, rocks, and birds.