So this is the Pantheon. It was built as a Roman temple in the 2nd century, replacing an earlier building.
In 609, the Byzantine emperor Phocas gave the building to Pope Boniface IV, who converted it into a Christian church and consecrated it to St. Mary and the Martyrs on 13 May 609: “Another Pope, Boniface, asked the same [Emperor Phocas, in Constantinople] to order that in the old temple called the Pantheon, after the pagan filth was removed, a church should be made, to the holy virgin Mary and all the martyrs, so that the commemoration of the saints would take place henceforth where not gods but demons were formerly worshipped.”
Inside I found it rather underwhelming. It’s just another church. I mean, it’s nice enough but I’m glad I didn’t have to pay or queue to get in.