The mosaic of the Cross as Tree of Life is the jewel of the twelfth century Basilica of San Clemente. But there are other wonders as well.
Surrounding the Cross, serving as frame for it, is the triumphal arch: also an extraordinary theological composition in mosaic.
At its apex is Christ as Pantocrator, flanked by the apocalyptic symbols of the four Evangelists. On either side are large figures of Peter and Paul, dressed in Roman tunics, in the company of and instructing Clement and Lawrence. Below them stand the prophets Isaiah and Jeremiah.
If the best of Greco-Roman culture offered only dim intimations of the glory to be revealed, the Hebrew prophets, read through the eyes of Christian faith, intuited the coming of God's Messiah.
A homily of Saint Melito of Sardis, in Holy Thursday's Office of Readings (Matins), recapitulates the Church's sense of the fulfillment realized in Christ:
There was much proclaimed by the prophets about the mystery of the Passover: that mystery is Christ, and to him be glory forever and ever.
He was led forth like a lamb; he was slaughtered like a sheep. He ransomed us from our servitude to the world, as he had ransomed Israel from the land of Egypt. He freed us from our servitude to the devil, as he had freed Israel from the hand of Pharaoh. He sealed our souls with his own Spirit, and the members of our body with his own blood.
It is he who endured every kind of suffering in all those who foreshadowed him. In Abel he was slain, in Isaac bound, in Jacob exiled, in Joseph sold, in Moses exposed to die. He was sacrificed in the Passover lamb, persecuted in David, dishonored in the prophets.
It is he who was made man of the Virgin, he who was hung on the tree. It is he who was buried in the earth, raised from the dead, and taken up to the heights of heaven.
He is the mute lamb, the slain lamb, the lamb born of Mary the fair ewe. He was seized from the flock, dragged off to be slaughtered, sacrificed in the evening and buried at night.
On the tree no bone of his was broken; in the earth his body knew no decay. He is the one who rose from the dead, and who raised man from the depths of the tomb.