A church was built here in the 4th century AD. The building that stands here now was begun on April 18, 1506 and was finished in 1626. Many Popes have been buried there. Although many people think St. Peter's is a cathedral, it is not, because it does not have a bishop. The pope is the Bishop of Rome, and although he usually uses St. Peter's as his main church, because he lives in the Vatican, his bishop's throne is in a different church, the cathedral of "Saint John Lateran". Large important churches like St. Peter's are often called basilicas. There are four ancient basilicas in Rome that were begun by the Emperor Constantine soon after he made Christianity the legal religion of the Roman Empire in the early 4th century AD (300s). The basilicas are "St. Peter's Basilica", "St. John Lateran", "Santa Maria Maggiore" and "St. Paul outside the Walls".