Archibasilica Sanctissimi Salvatoris ac Sancti Ioannis Baptistae et Ioannis Evangelistae ad Lateranum , the official Latin title of the Church (Archbasilica of the Most Holy Savior and Saints John the Baptist and John the Evangelist at the Lateran) was originally built as the Lateran Palace, a palace built on top of the remains of the New Fort of the Roman Imperial cavalry bodyguards. This fort was originally built in 193 and after Constantine the Great defeated Maenitus the fort was destroyed to the ground. The buildings on top of these ruins were first occupied by the Laterni family who served as administrators for several Emperors. Plautius Laternus though, was accused of treason and conspiracy to subvert the Empire by Emperor Nero and had the grounds confiscated.
Thus, when Emperor Constantine I married Fausta, the Palace and it's lands fell into his domain and was given to the Bishop of Rome. Saint Pope Sylvester I expanded the buildings and palace converting it to the Cathedral of Rome, the seat of the Popes. Saint Pope Slyvester I dedicated the archbasilica and Lateran Palace in 324, changing the name from House of Fausta to the House of God or Domus Dei.
Saint Empress Helena (mother to Emperor Constantine I) brought the stairs from the staircase that originally led to the praetorium of Pontius Pilate in Jerusalem to the palace and enclosed them in marble. These stairs were sanctified when Christ walked upon them during His Passion. Today the stairs are known as the "Holy Stairs" or Scala Sancta.
The Lateran baptistry stands somewhat apart from the Archbascilica and was founded by Pope Sixtus III and was for several generations the only baptistry in Rome. Legend holds that Constantine I was baptized here and many other Baptisteries followed the architectural example.
The archbasilica was rededicated by Pope Sergius III to Saint John the Baptist in the 900s, and by Pope Lucius II to John the Evangelist in the 1100s. The plaque on the front wall reads
"SACROS LATERAN ECCLES OMNIUM VRBIS ET ORBIS ECCLESIARVM MATER ET CAPUT"
"Most Holy Lateran Church, mother and head of all the churches in the city and the world"
Starting with Pope Militades, every Pope lived in the Lateran Palace until Pope Clement V moved the seat of the Papacy to Avignon. While in Avignon, the Palace and archbasilica fell into disrepair with two separate fires destroying much of the internal structures. When the Papal seat moved back to Rome, the Palace of the Vatican was built adjacent to the Basilica of Saint Peter. Pope Sixtus V spearheaded efforts to repair and reconstruct the palace and archbasilica. In the Square of the Lateran Palace is the Lateran Obelisk, the largest standing Obelisk in the world weighing over 455 tons. It was commissioned by the Pharaoh Thutmose II and sent to Rome under Constantius II for the Circus Maximus in 357. Pope Sixtus V found the pieces, rebuilt it, and placed it on a new pedestal in 1588.
Pope Innocent X continued the building with twelve niches. These Niches were left vacant until 1702 when Pope Clement XI and Benedetto Cardinal Pamphili commissioned statues of the twelve apostles -
Saint Paul - Pierre-Etienne Monnot
Saint Peter - Pierre-Etienne Monnot
Saint Simon - Francesco Moratti
Saint Jude Thaddeus - Lorenzo Ottoni
Saint Philip - Guiseppe Mazzuoli
Saint Thomas - Pierre Le Gros
Saint Bartholomew - Pierre Le Gros
Saint James the Lesser - Angelo de' Rossi
Saint Andrew - Camillo Rusconi
Saint John - Camillo Rusconi
Saint Matthew - Camillo Rusconi
Saint James the Greater - Camillo Rusconi
There are six Papal tombs in the archbasilica - Alexander III, Pope Sergius IV, Pope Clement XII, Pope Martin V, Pope Innocent III, and Pope Leo XIII. Pope Leo XIII was the last Pope not entombed in Saint Peter's Basilica.
The Feast of the Dedication has been observed since the 1100s, with November 9th being the Dedication of the Archbasilica of the Lateran (also known as the Dedication of the Basilica of the Most Holy Savior).