The Roma Pantheon that we see today was built by Hadrian between the years of 120 and 124 and was secularized and re-consecrated several times from the time of its creation.
It is ironic to think that the original Pantheon, built under the Roman Empire by Marcus Vipsanius Agrippa in 27 B.C. was a temple of the gods serving the seven deities of the seven planets. An inscription of Agrippa's name can still be found on the portico of the structure which was built with adjoining Roman baths and water gardens. The original temple was destroyed by fire in 80 A.D. before Hadrian had it built again, adding the original inscription to the new façade, and today is one of most remarkably well preserved monuments in Rome still boasting its original marble interiors and bronze doors not withstanding other losses that it took.
The Pantheon was given to Pope Boniface IV in 609 A.D. by the Byzantine emperor Phocas, who re-consecrated it as a Christian church named Santa Maria ad Martyres (Church of Mary and Martyrs) which has never changed.
The fact that it became a church saved the Pantheon form abandonment which unfortunately occurred to the majority of Rome's buildings during the medieval period, but nonetheless was stripped of its possessions when the Emperor Constance II came to Rome and pulled down all and any ornaments, including the bronze tiles, stripping off even the roof of the church. The valuables taken from the church were then sent to Constantinople.
During the reign of Pope Urban VIII between 1623-1644, he ordered that the valuable bronze ceiling be melted down ion order to make bombards for the fortification of Castel Sant'Angelo and it is believed that some of the bronze was also used by the famous artist Bernini to create the baldachin above the main altar in Saint Peter's Basilica.
The Pantheon has been used as a mausoleum since from the Renaissance with tombs there that include those of the great painter and sculptor Raphael, Annibale Caracci, architect Peruzzi and Italian kings Vittorio Emanuele II and Umberto I and his wife Queen Margherita.