Pietro Cavallini, The Apse of St. Paul Outside the Walls



19th century reconstruction (after the fire of 1823) of the 13th century original
Mosaic
Basilica of St. Paul Outside the Walls

Water flows from the right side of Christ's throne: compare Ezekiel's vision, "I saw water flowing from the Temple" in Ezekiel 47 (cached).  As in that chapter waterfowl feed in the stream and there are palm trees bearing fruit.  The water thus symbolizes the salvation that comes from Christ and leads the saved to the Paradise prophesied in Ezekiel.  This eschatological emphasis also explains the text that Christ is holding up for the viewer, which translates as "Come, you blessed of my Father, accept the kingdom prepared for you since the beginning of the world" (c.f. Matthew 25:34, from Christ's account of the Last Day).

But in addition to its eschatological focus, the upper register makes an important christological statement.  The scroll held by St. Peter (to the right of Christ) is from the important passage in Matthew 16:16 where Peter says, "You are the Christ, the Son of the living God."  To the left of Christ, St. Paul holds the equally important text from Philippians 2:10 boldly asserting the divinity of the Christ: "At the name of Jesus, every knee should bend, in Heaven, on earth, and in the regions below."  (This text is in turn a reference to Isaiah 45:24, "every knee shall be bowed to me, and every tongue shall swear")  And Andrew's scroll (far right) makes him a witness to Jesus' divinity through the apostle's own death on the cross: "Blessed Andrew when he hangs on the cross will pray to the Lord Jesus Christ."  Jesus is both Messiah and "Lord," according to this iconography, and worshiping him is worshiping God.

The lower register continues both the eschatological and the christological emphases of the original mosaic.  Fruited palm trees also figure here, continuing the paradisical symbolism of the upper mosaic.  The cross in the center represents Christ, with the instruments of his Passion solemnly arrayed below the crosspiece.  The two angels standing left and right of the cross hold scrolls bearing the first two phrases of the Gloria in Excelsis, a hymn sung in solemn masses and based on the angels' announcement to the shepherds in Luke 2.  In its full form, the Gloria is a hymn to the Trinity, but here, with the saints facing toward the cross in the center, it has been adapted to the praise of the Son. The selection and arrangement of the phrases also focuses on the person of Christ: "Only-begotten," "Son of the Father," and so on.

Left and right of the central figure of Christ in the upper register are the Greek abbreviations IC (for "IESUOS") and XC (for "CHRISTOS").  The large figure at the far left is St. Luke.  The small white figure bowing at Christ's feet is Pope Honorius III, at whose order the work was done. 

Left to right, the figures in the lower register are (with the words on their scrolls):
 Also see detail photographs of the following:
St. Luke
St. Paul
Christ
St. Peter
St. Andrew

More of the Cross
More of the Apostles and Evangelists:
Photographed at the site by Richard Stracke