The Apse at Monreale Cathedral: Detail, Christ Pantocrator
This is a typical Christ Pantocrator, a half-height portrait of Christ as in Heaven, his right hand blessing the viewer from the apse of the church while his left hand holds an open book. In this case, the book is open to John 8:12 ("I am the light of the world. Whoever follows me will not wander in darkness.") The left page of the book has the verse in Latin and the right has it in Greek.
The Greek words on the left and right of Christ's head, ΙΗΣΥOC XPIΣTOC O ΠANTOKPATOP, mean "Jesus Christ the Pantocrator [Ruler of All]."
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Read more about Christ in Majesty.
Photographed at the cathedral by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.