The Basilica of St John Lateran Is one of the four most important of the Seven Pilgrim Churches of Rome, along with St Peters Basilica in the Vatican (San Pietro in Vaticano), San Paolo Fuori Le Mura (St. Paul Outside the Walls), and Santa Maria Maggiore (Saint Mary Major). Faithful who traveled the ancient pilgrimage route, Via Francigena, that runs from Canterbury, England, and through France and Switzerland to Rome were expected to visit at least these four churches at the end of their journey.

Rome's Cathedral of St. John is dedicated to both John the Baptist and John the Evangelist. Its name is Basilica di San Giovanni in Laterano in Italian, and it was originally built by Constantine the Great in the fourth century as the first Christian church built in the city. The "Lateran" in the name refers to the site, which once was the location of the Laterani family palace. Constantine envisioned this as Rome's Cathedral (also known as the "Mother of all Churches") and it maintained its exclusive standing until the Popes returned to the city in 1377 after their long residence in Avignon, France. The Vatican then became their official residence, as it was situated in a drier and healthier location. There is an obelisk from the Temple of Karnak in Luxor, Egypt standing in the square in front of the church.

Rome's Cathedral was first dedicated by Constantine in the year 318 AD. Only the baptistery still survives from this time period in its original form. The church became the Basilica di San Giovanni in Laterano when Pope Sergius III dedicated it to John the Baptist in the tenth century. Pope Lucius II added John the Evangelist in the twelfth century. The Basilica of St John Lateran was almost completely destroyed by an earthquake in 896. A new church was built on the site, and it was extensively damaged by fires, especially in the years 1308 and 1360. When the Popes returned from France, extensive rebuilding and remodeling began, lasting until 1735, resulting in the magnificent structure we see today. The famous façade was designed by Alessandro Galilei, who also designed stately homes in Ireland, England, and churches in Florence, Italy. The façade is fairly stark by the Roman standards of the time, and it caused quite a bit of controversy because of this. It later gained admirers (especially those in Georgian England and Ireland) in northern Europe, and is greatly admired today.

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While the Basilica of St John Lateran is well outside the boundaries of Vatican City (which is actually a separate, independent country), special dispensation allows it to be part of the Holy See. It was, in fact, the 1929 Treaty of Lateran, the agreement between the Catholic Church and the country of Italy, that granted full sovereignty to the Holy See. This is also the case for several other sacred churches in Rome. As Rome's Cathedral and the "Mother Church" that is the oldest and most important church in Catholicism, the Basilica di San Giovanni in Laterano is still an active church. Some of the best examples of fourth century art and architecture are contained in it, including its central Roman bronze doors that came from the Senate Houses in the Imperial Forum, and the statue of Constantine the Great, which was found in the Baths of Diocletian. A cedar table within the ornate altar is reputedly the actual table used by the Christ during his Last Supper in Jerusalem. Other sacred relics include a reliquary said to contain the heads of Saints Peter and Paul and what is said to be part of St. Peter's communion table. The latter is on the High Altar, which can only be used by the Pope.