As we remember from high school history, Constantine was the emperor of the Western Empire from 306 to 337 AD.  His father had been Roman emperor from 305-306. When Constantine took over, he had one major enemy, Maxentius, who was son of the Roman
Emperor Maximiari, of the Laterani family.(285-305). In 312, Constantine decidedly defeated Maxentius at the battle of Mulvian Bridge. It is said that Constantine, a pagan, had turned to Jesus for help in winning and that all his soldiers went into battle with the cross on their shields. Within a year, Constantine, now undisputed Emperor, signed the Edict of Milan, making Christianity legal throughout the empire.

In thanksgiving for the triumph, Constantine gave the Church one of the palaces he had received as a dowry when he married Fausta Laterani, the sister of Maxentius. He presented the palace and the horse stables, in the middle of Rome to Pope Melchiade. The next year, Constantine had the stables taken down and ordered a church built there in honor of Jesus the Savior. You can still see the seven meter high statue of Jesus on top of the building. Constantine also dictated that other basilicas be built over tombs of the saints around Rome.

This was the primary church in Catholicism for 1200 years. Over time, the basilica has been ransacked, burned and suffered an earthquake. The exterior has been updated numerous times. The interior has remained similar in design through out this time and improvements often used old building pieces.

You can read more about this basilica at https://www.ewtn.com/library/CHRIST/LATERAN.HTM.

“As often as we celebrate the dedication festival of an altar or church if we assist with faith and attention, living holily and religiously, that which is done in temples made with hands is done also in us by a spiritual building.”
St. Augustine