Santi Nereo e Achilleo:
​Mosaics

Map
Interior of the church of Ss Nereo e Achilleo, Rome
Apse
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 apsidal arch, which survives from the time of Pope Leo III (r. 795-816) and depicts the Transfiguration of Christ. 
Mosaic of the Transfiguration, church of Santi Nereo e Achilleo, Rome
The Transfiguration
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 did shine as the sun, and his raiment was white as the light' (Matthew 17:2, KJV). 

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 beloved son, in whom I am well pleased '. (Matthew 17:5, KJV). The three apostles fell prostrate before the vision and we see them averting their faces. 

At each end of the mosaic is a depiction of the Annunciation and the Virgin and Child.  ​