Praised be Jesus and Mary, now and forever. 
Today we celebrate the dedication of the Basilica of St. John Lateran, which is the Pope's Cathedral. As such, it is the mother and the head of all the churches of Rome and of the world. From it, an ever-flowing river of graces goes out into all the world, as in Ezekiel's vision. Its full name is the Archbasilica of the Most Holy Saviour and of St. John the Baptist and the Evangelist in the latter. So it's primarily dedicated to Christ our Saviour and in addition to the two best known St. Johns. The original basilica was built by Emperor Constantine. The land on which it was built used to belong to the Laterani family, but in AD 65 it was confiscated because the owner was implicated in a conspiracy against the Emperor Nero. After that it was imperial property and put to various uses. It came into Constantine's hands in 307 when he married Fausta, his second wife, in order to cement a political alliance. She was the daughter of the former Emperor Maximian and the property was part of her dowry. Five years later in 312 Constantine won the Battle of the Milvian Bridge by putting the sign of the cross on his soldiers' shields. This was the sign he had seen before the battle in a dream in which he was told, In hoc signo vincis, by this sign you will conquer. Until that point he was not a Christian, but his mother was.
Thereafter he was a supporter of the church and he was eventually baptized. In 313 he legalized Christianity by the Edict of Milan, putting an end to the persecution, at least in the West. That same year a synod of bishops gathered at the Lateran and condemned the Donatist schism. So already at that early date the Pope seems to have had the occasional use of Fausta's palace, or the palace that was Fausta's dowry. Now that Christianity was legal and had the Emperor's favor, suitable buildings were needed. The type of building known as a basilica was the model for building large churches. It's literally a royal hall, a kind of building that was designed for public assemblies. And so a type of building that architects knew how to build and that could accommodate a large assembly of the faithful. The one built as a church on the land that once belonged to the Laterani family is called the Lateran Basilica. It was consecrated in 324, so 11 years after Christianity was legalized, and it was dedicated to our Savior. In that time it has always been the Pope's Cathedral. The Cathedral takes its name from the cathedra, or throne of the Bishop of Rome, a symbol of his teaching authority, which was located in the apse of this Cathedral. So we talk about dogmas defined ex cathedra, defined with that authority that is proper to the Roman pontiff. For nearly a thousand years this was the official residence of the Pope, so Lateran, during that time, had a meaning like Vatican does today. An octagonal baptistery was built next to the church on the site of the paths of the old Lateran Palace. The ancient baptisteries were separate from the churches because it is by baptism that we enter the church. So first you have to be baptized and then you can come into the church. This baptistery was rebuilt a century later by Pope Sixtus III, and in the 460s, so in the 5th century, Pope Hillary added two chapels to the baptistery, one dedicated to St. John the Baptist, kind of an obvious choice for a baptistery, and the other dedicated to St. John the Evangelist. So now here we see already this association with the two St. Johns in addition to the Savior, although the St. Johns are in the chapels of the baptistery. The Lateran complex grew. There were various buildings all connected, and by the Middle Ages, this complex included not only the Basilica, Baptistery and Palace, but also a hall for councils and the archive and the library. In the palace was the Pope's Chapel, which became known as the Sancta Sanctorum, the Holy of Holies, because of the exceptional relics it contained. It was said to be the holiest place on earth because of those relics. This chapel is the only part of the palace that is left. It's now in the Scala Sancta Church, the church where the holy stairs are venerated, those stairs which are said to have been the very stairs that were between Pontius Pilate and Christ when Christ was brought before Pilate. On the stairs there are spots of blood that are venerated by the pilgrims that go up the stairs on their knees. So in the Scala Sancta Church there's now the Sancta Sanctorum, which was the Pope's Chapel. There was the liturgy of this chapel that was chosen by St. Francis and by the Franciscans after him for the liturgy of the Franciscans, and thus they spread the Roman liturgy or this particular form of the Roman liturgy throughout the world.
The church, the Basilica, was damaged and repaired more than once over the centuries. Earthquakes came, barbarians came and either raided it for valuable items or just damaged it in other ways. In 896 an earthquake did heavy damage to the Basilica and so it had to be rebuilt by Pope Sergius III who in 905 rededicated it to St. John the Baptist. So that from then now it's called St. John Lateran. But it retains its original dedication to our Savior and it's also the association with St. John the Evangelist from that chapel that was added to the baptistry. Over the centuries many synods and councils have met there. In 649 an important synod condemned the heresy of monoenergism and defined the dogma of Mary's perpetual virginity. 
There are five ecumenical councils of the Lateran, most held during the Middle Ages, but one also in the 1500s. The most important of these councils is the fourth which was held in 1215. This is said to be the occasion on which St. Francis and St. Dominic met. The word transubstantiate was used by the council to define what happens when the Eucharist is consecrated. This council also made confession obligatory at least once a year so as to whether they received communion at Eucharist. 
In 1308 the Lateran complex burned and in the following year the Popes moved to Avignon. They did have the Lateran repaired, at least parts of it, but in 1360 while they were still at Avignon it burned again. This time it was repaired by Pope Urban V who returned to Rome from Avignon but only stayed for two or three years. When the papacy definitively returned to Rome with Gregory XI in 1376, the Popes began to reside instead at St. Peter's in the Vatican. They no longer lived in the Lateran Palace. The Vatican was in a healthier part of the city, but the Lateran Basilica remains the cathedral and retains its preeminence over all other churches in Rome and in Christendom, including St. Peter's.
The Popes continued to maintain the basilica and the baptistry because they used those, but not so much the palace which they didn't live in. So the palace gradually fell into further disrepair and of course it had been unused by the Popes for about 60 years when they were in Avignon.
The last major renovation of the basilica and baptistry was under Blessed Pius IX. Eventually that palace that had fallen into disrepair was demolished by Sixtus V who did a lot of building projects around Rome. He built a new Lateran Palace which is now where the Curia of the Diocese of Rome resides, and there too close to the beside that is the Lateran University as well. So this is the mother and head of all the churches in the world. It is the source from which graces flow for all the churches and thus we rightly celebrate it today.
Praised be Jesus and Mary, now and forever.