“The apse mosaic always signifies paradise,” Dr. Bentz explained on one of our recent Virtual Pilgrimages. This is why they all have this golden background, which signifies heaven or heavenly glory. Paradise is also a garden with plants and animals, so there are plenty such symbols in the apse mosaics.
In this particular mosaic, there are four saints surrounding Jesus on his throne. From left to right these saints are St. Luke, St. Paul, St. Peter, and St. Andrew. In addition to these, we see a tiny little Pope at the foot of Jesus. The little figure is Pope Honorius III, who commissioned this apse mosaic and whose title in inscribed in the top center circle of the arch. The reason why Pope Honorius is so tiny has to do with medieval symbolism. The more important someone is, the bigger they are portrayed. Even though the Pope is an important figure, in comparison with Jesus, the Pope is pretty insignificant. Thus, the tiny portrayal.
Taking a closer look at Jesus, we find that he is making a gesture with his hand. This sign, the joining of the thumb and the ring finger, symbolizes the Incarnation. The thumb and the ring finger are divinity and humanity coming together in Jesus Christ. It is also to be noted that every person in this mosaic, except Pope Honorius, is holding a scroll. If you read them, these scrolls are all testaments to the divinity of Christ.
At the bottom of the mosaic, there are more saints surrounding Jesus. In this lineup, however, Jesus hides behind the instruments of his passion: the cross, the nails, and the crown of thorns. This is reminiscent of how we see Christ on earth. We can only see him through the instruments of his passion, but in heaven we will be able to see him directly in all his splendor.
Resources: our Virtual Pilgrimage to St. Paul’s Outside the Walls (watch on YouTube here)