Detail of the San Vitale Hospitality of Abraham Mosaic: Angels with Clipeus

This is visually and symbolically the central element in the Hospitality of Abraham mosaic. The design adapts a pattern from classical sarcophagi, in which flying erotes hold a clipeus with a portrait of the deceased. Here is an example from the 2nd century:

Roman sarcophagus with flying erotes holding a clipeus portrait, circa 190-200 A.D. (Source: Metropolitan Museum of Art, public domain CC0.)

Thus we may think of the jeweled cross inside the clipeus as representing Christ himself, not simply the cross on which he died. A similar use of a cross to represent Christ is seen a few miles away in the apse of Sant'Apollinare in Classe.

I have not yet discovered the significance of the two omega-shaped pendants hanging from the crossbar. It is not uncommon in this era for the cross to be combined with an alpha (Α) and an omega (Ω or ω) because of Christ's words "I am the alpha and the omega, the beginning and the end" (Revelation 1:8, c.f. 21:6, 22:13). A medieval manuscript illumination in the Metropolitan Museum has an Α and an ω as pendants from the crossbar. But why does this image have two omegas instead?

Conceivably, they could be anchor symbols, like this early very early sign from Rome. The anchor was an early Christian symbol for the Cross (Sill, 128) and was actually more common than the cross in the earliest Christian art (Catholic Encyclopedia, s.v. "Anchor [as Symbol]"). Not only was it vaguely cross-shaped itself, but it referenced Hebrews 6:18-19, "we…hold fast the hope set before us, which we have as an anchor of the soul, sure and firm."

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The mosaic was photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.