The church of Santi Nereo e Achilleo was founded in the 6th century, but largely rebuilt at the end of the 16th century. Thankfully, the rebuilding didn't destroy the beautiful mosaic on the choir arch, which survives from the time of Pope Leo III (r. 795-816) and depicts the Transfiguration of Jesus.
The transfiguration is the moment when Christ manifested his divine nature to the disciples Peter, John and James. He took them up a mountain (traditionally Mt Tabor in Galilee) and in their presence became transfigured: his face shone like the sun and his clothes became dazzling white. Moses and Elijah appeared on either side and conversed with Christ, while a bright cloud overshadowed them and a voice from heaven declared: 'This is my son'. The three apostles fell prostrate before the vision and we see them averting their faces.
On either side of the Transfiguration are images of the Annunciation and the Virgin and Child.