Inside the basilica, above the great triumphal arch, one finds the following mosaic:
Of this image Pope Benedict XVI - then-Cardinal Ratzinger - has written:
In the middle, above the arch of triumph, to which the aisles lead, where we would expect the birth of Christ to be represented, we find instead only an empty throne and on it a crown, a ruler's mantle, and the cross. On the footstool lies the bundle of history like a pillow bound together with seven red threads. The empty throne, the cross, and history at its feet - that is the Christmas image of this church, which claimed and claims to be the Bethlehem of Rome. Why? If we wish to understand the image we have first to recall that the arch of triumph stands above the crypt that originally was built as the replica of the cave of Bethlehem in which Christ came into the world. Here the relic was and is still venerated that according to tradition is the manger of Bethlehem. So here the procession of history, all the splendor of the mosaics, is abruptly pulled down into the cave, into the stable. The image falls down into reality. The throne is empty, for the Lord has come down into the stable. The central mosaic, to which everything leads, is likewise only the hand that is extended to us so that we might discover that leap from the images to reality. The rythym of the space pulls us into a sudden change when it thrusts us out of the brilliant heights of ancient art in the mosaics directly into the depth of the cave, of the stable. It seeks to lead us into the transition from religious aestheticism to the act of faith (Images of Hope: Meditations on Major Feasts, 18-19).The mosaic is a reminder to us that "Jesus Christ, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men" (Philippians 2:5-7).
In the homily he preached at his inaugural Mass of his pontificate, Pope Benedict XVI put it slightly different in these moving words:
The human race – every one of us – is the sheep lost in the desert which no longer knows the way. The Son of God will not let this happen; he cannot abandon humanity in so wretched a condition. He leaps to his feet and abandons the glory of heaven, in order to go in search of the sheep and pursue it, all the way to the Cross. He takes it upon his shoulders and carries our humanity; he carries us all – he is the good shepherd who lays down his life for the sheep.At the same time, though, the empty throne also calls to mind the fact that the one who left the throne to be born in Bethlehem will also be enthroned forever around whom the faithful will be gathered to endlessly cry out, "To him who sits upon the throne adn to the Lamb be blessing and honor and glory and might for ever and ever" (Revelation 5:13)!