St John Lateran, aka the Archbasilica of Our Saviour, was built by the Emperor Constantine, and consecrated by Pope Sylvester on November 9, 324. It is Rome's (and hence the Pope's) official Cathedral, and ranks as one of the four papal basilicas.
It has a particular significance for Benedictines, as monks fleeing the destruction of Monte Cassino after St Benedict's death established a monastery there, their stories of the founder persuading the future Pope Gregeoy I to become a monk, and to record the life of the saint.
Unfortunately the records of the Church (and subsequent monasteries) are sparse, not least because the basilica was destroyed first by the vandals in 460, and again by an earthquake in 896. And it burnt down in 1307 and again in 1361... The current Church largely dates from the seventeenth century, with some extensive remodelling completed in 1735.
The dedication to St John reflects the monastery established there which served the Cathedral, dedicated to St John the Baptist and St John the Evangelist.