Look up at the apse and you will see one of the most breathtaking treasures of early Christian art: a sixth-century mosaic that has survived since the reign of Pope Felix the Fourth. Unlike the later golden backgrounds you might see in other Roman churches, this mosaic features a deep, vibrant blue sky—a hallmark of the Roman-Byzantine tradition. In the center, Christ descends through red and blue clouds in a scene known as the Parousia, or the Second Coming. He is dressed in a magnificent golden toga, flanked by Saint Peter and Saint Paul, who are presenting the brothers Saint Cosmas and Saint Damian. If you look closely at the far left, you will see Pope Felix himself holding a small model of this very church—a classic signature of the man who commissioned this masterpiece nearly 1,500 years ago.