Santi Cosma e Damiano, which was built in 527 by Pope St Felix IV (r. 526-530), was the first church to incorporate in its structure the remains of ancient Roman buildings, namely the audience hall (or library) of Vespasian’s Temple of Peace and the Mausoleum of Romulus (now thought to be the Temple of Jupiter Stator).
The mosaic in the apse is one of the most beautiful in Rome. Christ stands before a background of colourful clouds, dressed in a golden toga. The toga is inscribed with the letter iota, the tenth letter of the Greek alphabet, possibly a reference to the Ten Commandments. In his left hand, he holds a scroll, while his right hand is uplifted, palm open to the congregation.
Christ is accompanied by St Peter and St Paul, who are flanked by St Cosmas and St Damian, two Arab physicians (reputedly twins), who were martyred towards the end of the 3rd century. On the far right is St Theodore and on the far left Pope St Felix IV holds a model of the church. The latter figure only dates back to the 1630s when part of the mosaic was reworked.
Directly below Christ, at the bottom of the mosaic, the Agnus Dei (Lamb of God) stands on a mount, from which flow the four rivers of Paradise. The Agnus Dei is flanked by twelve sheep representing the twelve apostles. At each end of the row of sheep are the cities of Bethlehem and Jerusalem.
In the centre of the triumphal arch is the Agnus Dei, lying, not standing, on the throne. The seven candlesticks to either side represent the Seven Lamps of Revelation. The half figure of an angel on the far left is a symbol of St Matthew, while on the far right the eagle is a symbol of St John the Evangelist.
In 1632 Santi Cosma e Damiano was restored and remodelled by Orazio Torriani and Luigi Arrigucci, on the orders of Pope Urban VIII (r. 1623-44), whose heraldic bees are prominently displayed throughout the church.
Unfortunately, the remodelling, which involved raising the floor and adding side chapels, damaged some of the mosaics on the triumphal arch.