The Transfiguration Mosaic at Sant'Apollinare in Classe

6th century
Basilica of Sant'Apollinare in Classe, Ravenna

This image is discussed in Margaret E. Frazier's "The Christian Realm: Apse Themes, in Weitzmann. The cross represents the transfigured Christ. To its left and right are Moses and Elijah, respectively. The figure below with the sheep is St. Apollinaris.

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Detail photographs:

More of Saint Apollinaris
More of the Transfiguration
More of sheep as symbols in Christian art

Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.