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 flanked by St Peter and St Paul, who are both clad in purple-banded white togas (that of St Peter also sports the letter iota). Each saint holds a hand towards Christ as they present Cosmas and Damian, two Arab physicians (reputedly twins), who were martyred in 303. Each martyr carries a crown, but the figure on the right also carries a small red satchel. On the far right stands St Theodore, carrying a crown, while on the far left stands Pope St Felix IV holding a model of the church. His figure only dates back to the 17th century when part of the mosaic was reworked.
Below the word Iordanes (River Jordan) 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 either end 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 the Evangelist, 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.
The remodelling, which involved raising the floor and adding side chapels, damaged some of the mosaics of the triumphal arch presence of only two Evangelist symbols indicates that the mosaic has been cut back at some point.
The ancient brick wall, to the left of the main entrance of the church, once sported a monumental scaled map of the city of Rome, the Forma Urbis Romae. The map (203-211 CE) was 18 m high by 13 m wide and made up of 151 slabs of marble. Sadly, only ten percent of it has survived, in the form of more than 1100 fragments.