The work of St. Peter’s Basilica officially began on April 18, 1506 under the pontificate of Pope Julius II. However, the history of the complex is much older, dating back to the 4th century AD. when the edict of Milan (or edict of Constantine) was signed. This agreement provided for freedom of worship for all citizens of the Eastern and Western Roman Empire, and was a key step in the spread of Christianity. After 313 (year of the edict) churches and basilicas were built to host the various religious services.
The Basilica of San Pietro, symbol of Christianity, was in fact built in 333 on the site of Circo Caligola where Pietro was crucified.