The Apse at Sant'Apollinare in Classe: Detail, The Cross
Jesus Christ is represented by the cross itself, which the mosaic labels SALUS MUNDI, "the salvation of the world," by his face in the middle, and also by the Α and ω left and right of the crosspiece. The Greek letters refer to Revelation 1:8 et pass., "I am the Alpha and Omega, the beginning and the end, saith the Lord God."
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Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.