In his Angelus Address
this past Sunday Pope Benedict XVI spoke about the feast of All Saints and linked it to a very visible Roman landmark.
Originally a pagan temple, the Pantheon was dedicated to all Roman gods. Marcus Agrippa, a consul in Rome, had the temple built in 27 B.C. The temple was caught in two fires and rebuilt each time. It was completed in it's current form under the Emperor Hadrian (interestingly enough Hadrian's tomb, the Castel Sant'Angelo, was built with a similar circular design). The Pantheon served as a temple to all Roman gods until the year 609 when the Byzantine Emperor Phocas gave the temple to Pope Boniface IV who consecrated it, turning it into the Church of St. Mary and the Martyrs on May 13 of 609. That day became the feast of All Martyrs and in the process also replaced the pagan Feast of the Lemures which fell on the same day.
Pope Gregory III later created an oratory at St. Peter's to house the relics of the holy apostles, saints, martyrs, and confessors. With the founding of that oratory he expanded the feast of All Martyrs to include all saints and moved the feast day to November 1st. The conversion of the pagan temple into a Christian Church probably saved it from being abandonded and falling into disrepair during the Middle Ages. That did not, however, save the church from being stripped of the copper that lined the inside of the dome, thanks to the Barberini pope Urban VIII. The Barberini family were infamous for stripping Roman monuments of their valuable parts, so much so that Romans still say "What the Barbarians didn't destroy, the Barberini did."
Since the Renaissance the Pantheon undergone some restoration and serves as a tomb. The painter Raphael is entombed here along with King Vittorio Emanuele II and King Umberto I of Italy as well as Queen Margherita.