The Apse Mosaic in Sant'Ambrogio Basilica, Milan
Original segments: 5th through 8th centuries
Restorations: 17th and 20th centuries
Basilica of St. Ambrose, Milan
Christ sits enthroned between Saint Gervasius (on the right) and Saint Protasius (on the left), each of whom holds a hand cross in the left hand. St. Michael is above Protasius, St. Gabriel above Gervasius. The open book in Christ's hand reads EGO SUM LUX MUNDI, "I am the light of the world." Those words and the saints' labels are in Latin, but the labels for Christ and the angels are in Greek.
Although heavily restored in the 17th and 20th centuries, parts of this mosaic date back to the 5th and 8th.
Read more about SS. Gervasius and Protasius and about Christ in Majesty.
Source: Giovanni Dall'Orto via Wikimedia Commons