When Christianity was legalized in the Roman Empire, instead of tearing down Roman temples, Christians exorcized them and rededicated the buildings as Christian churches. An instance of this occurred on May 13, 610, when Pope Boniface IV consecrated a former Roman temple, giving it the new title of “St. Mary and the Martyrs.”
Built by Emperor Agrippa and completed around 126 AD, this temple was previously dedicated to all the pagan gods. It is known today as the “Pantheon” in reference to this original dedication and remains an architectural marvel of the ancient world.
When the Pantheon was first consecrated as a Christian church many relics of Roman martyrs were brought there from the catacombs, which helps explain its original name. Later on the title of the church was broadened to include “St. Mary and All the Saints,” but the feast commemorating its dedication remained on May 13.
Then on November 1, 735, Pope Gregory III dedicated a chapel in St. Peter’s Basilica to a variety of saints, making it a privileged feast day in the city of Rome. Shortly thereafter Pope Gregory IV established November 1 as a holy day of obligation in the universal Church dedicated to All Saints. To further cement the day, Pope Gregory VII transferred the Pantheon’s feast from May 13 to November 1, combining the two dedications to emphasize it and give it even more solemnity.