Built around 126 AD under Emperor Hadrian as a temple to all the gods, the Pantheon survived the fall of Rome largely because Byzantine Emperor Phocas granted it to the Pope, who reconsecrated it as a church. This conversion is a major reason the building remains the best-preserved structure of ancient Rome, its massive unreinforced concrete dome still the largest of its kind in the world.
By the early seventh century, Rome had shrunk dramatically from its imperial peak, and countless ancient monuments had been stripped for building material, burned for lime, or simply collapsed from neglect. The Pantheon, with its extraordinary coffered concrete dome and open oculus, faced the same fate as so many pagan temples across the crumbling empire.
Pope Boniface IV petitioned the Byzantine Emperor Phocas, who nominally still ruled over Rome from Constantinople, for permission to convert the temple into a Christian church. Granted the building, Boniface consecrated it around March 609 as Santa Maria ad Martyres, reportedly after having 28 wagonloads of martyrs' bones transferred there from the catacombs.
The reconsecration proved to be an extraordinary act of architectural preservation almost by accident. Because it functioned continuously as a church, the Pantheon escaped the systematic quarrying and demolition that destroyed so much of ancient Rome, and today it stands as the most intact major building surviving from antiquity, still awing visitors with an oculus that lets rain fall straight onto its marble floor.