Discover Archbasilica of St. John Lateran (Cathedral)

The Archbasilica of Saint John Lateran is the cathedral of the Diocese of Rome and the seat of the bishop of Rome — the pope. It is the only archbasilica in the world, and Turismo Roma calls it, by definition, the mother of all the churches of the world. It is not at the Vatican: it stands about four kilometres to the south-east, near the Caelian hill, and it outranks Saint Peter's.

It is also the oldest of the four major papal basilicas — the oldest public church in the city of Rome and the oldest basilica in the Western world. The two sources date it differently: Turismo Roma says the basilica stands where Constantine built one around 314, consecrated by Pope Sylvester I in 324, or 318; the reference work gives the foundation simply as 324.

You enter through the doors of the Roman Senate. In 1660, Alexander VII had the bronze leaves of the ancient Roman Curia brought from the church of Sant'Adriano al Foro Romano; they are the great central portal today.

And a cavalry barracks lies directly beneath the nave. The basilica stands over the Castra Nova equitum singularium, built by Septimius Severus in AD 193 for the imperial horse guard — the guard that fought for Maxentius at the Milvian Bridge and was abolished and its fort demolished afterwards. Its remains were found in the 20th century during restoration of the cosmatesque pavement.

What you walk into is mostly Borromini's, who in 1650 completely rebuilt the central nave and the aisles, and Alessandro Galilei's, whose facade was completed in 1735 under Clement XII, with the twelve Apostles standing in Borromini's niches.

Two things here are worth the detour on their own: the museum of liturgical furnishings and the cloister, a masterpiece of cosmatesque work by Pietro Vassalletto — of the same family of Roman marble workers who made the cloister at Saint Paul Outside the Walls.

And the president of France is a canon of this basilica. The head of the French state is ex officio the First and Only Honorary Canon, a title held since Henry IV.

Hours: basilica 07.00–18.30, cloister 09.00–18.00, baptistery 07.00–12.30 and 16.00–19.00, museum 10.00–17.30. Turismo Roma publishes no admission price and warns the hours may vary.

Almost nothing you look at is old, and the two oldest things are the floor and the door. The building deteriorated in the Middle Ages and was badly damaged by two fires in the 14th century, was rebuilt in the late 16th century under Sixtus V, renovated inside in the late 17th, and given its facade in 1735 under Clement XII.

The nave is Borromini's. In 1650 he completely rebuilt the central nave and the side aisles, working on the structural changes Eugene IV had made between 1431 and 1447. His solution was to wrap the old columns into massive piers and cut great niches into them — and in the 18th century the twelve Apostles were set into those niches.

The front is Galilei's and it is a civic facade, not a church one. Turismo Roma records the facade finally completed in the 18th century with the new front by Alessandro Galilei. Its inscription names the builder rather than the saint: Clemens XII Pont Max Anno V Christo Salvatori In Hon SS Ioan Bapt et Evang — Clement XII, in the fifth year of his pontificate, to Christ the Saviour, in honour of Saints John the Baptist and the Evangelist. Because Christ the Saviour is the primary dedication, the titular feast is 6 August, the Transfiguration.

The central portal is a Roman antiquity in use. In 1660 Alexander VII had the bronze leaves of the ancient Roman Curia moved here from Sant'Adriano al Foro Romano.

The floor is medieval and sits on a barracks. The cosmatesque pavement was restored in the 20th century, and that restoration found the remains of the barracks of the equites singulares — the Castra Nova built by Septimius Severus in AD 193, whose substantial remains lie directly beneath the nave.

The cloister is the finest thing on the site and it is not by any of these architects. It is Pietro Vassalletto's, a masterpiece of cosmatesque art — the same family of Roman marble workers who built the cloister at Saint Paul Outside the Walls. It holds architectural elements, sculptures and ornaments from the ancient basilica, which is where most of what survived the fires ended up.

The site was cleared by a political reprisal before it was ever built on for the church. The land belonged to the Laterani until Nero confiscated it after the Pisonian conspiracy. Septimius Severus put the Castra Nova equitum singularium on part of it in AD 193. When Constantine defeated Maxentius at the Milvian Bridge in 312, the horse guard — which had fought for Maxentius — was abolished and the fort demolished. In 313 Constantine gave the ground to Pope Miltiades.

The first church was a hall for assemblies turned into a church. Turismo Roma describes it as born as a building for public meetings and the administration of justice, transformed with the spread of the new faith into a structure able to hold a great number of the faithful. Turismo Roma dates Constantine's basilica to around 314 and the consecration by Sylvester I to 324, or 318; the reference work gives 324.

Then it was rebuilt, repeatedly, because it kept being destroyed. The Vandals stripped it in 455; Leo III rebuilt the ceilings in the 9th century; it deteriorated through the Middle Ages and was badly damaged by two fires in the 14th century; and after 1378 the popes abandoned it for the Vatican in very poor condition.

The recovery was done in four campaigns by four architects. Martin V remade the pavement and commissioned Gentile da Fabriano and Pisanello (1426–1431); Eugene IV made structural changes (1431–1447); Giacomo della Porta renewed the transept for the Jubilee of 1600; Borromini rebuilt the nave and aisles in 1650, relying on Eugene IV's work; and Alessandro Galilei finished the facade in the 18th century, the reference work dating it 1735.

The last additions are a door and a discovery. Alexander VII installed the Curia's bronze doors in 1660; the barracks were found in the 20th century beneath the cosmatesque floor; and the Holy Door by Floriano Bodini was inaugurated for the Jubilee of 2000.

Is this bigger or more important than Saint Peter's? It outranks it in status. It is the cathedral of the Diocese of Rome and the seat of the bishop of Rome, the only archbasilica in the world, and holds the title of ecumenical mother church. Turismo Roma calls it the mother of all the churches of the world.

How old is it? The reference work gives it as founded in 324, the oldest public church in the city of Rome and the oldest basilica in the Western world. Turismo Roma dates Constantine's basilica to around 314 and the consecration by Sylvester I to 324, or 318.

Is it in the Vatican? No. It lies about four kilometres from Vatican City, but as Holy See property it has extraterritorial status from Italy under the Lateran Treaty of 1929.

What are the hours? Basilica 07.00–18.30; baptistery 07.00–12.30 and 16.00–19.00; sacristy 08.00–12.00 and 16.00–18.00; cloister 09.00–18.00; museum 10.00–17.30; historical archive Monday to Friday 08.30–13.00.

How much does it cost? Turismo Roma publishes no admission price for any part of the complex. That is a gap in the source, not a guarantee that the cloister and museum are free.

Why is it called Saint John's if it is dedicated to Christ? Christ the Saviour is the primary dedication — which is why the titular feast is 6 August, the Transfiguration. Sergius III added John the Baptist in the 9th century and Lucius II added John the Evangelist in the 12th.

Is the French President really a canon here? Yes. The president of the French Republic is ex officio the First and Only Honorary Canon, a title French heads of state have held since Henry IV.

What is under the floor? The Castra Nova equitum singularium, the imperial cavalry barracks built by Septimius Severus in AD 193, whose substantial remains lie directly beneath the nave.

How long should I allow? About an hour and a half.