The Lateran Baptistery which stands adjacent to St John Lateran’s Basilica in Rome and is also known as ‘San Giovanni in Fonte’ was the first Baptistery to be constructed after freedom of worship was granted to Christians by the Emperor Constantine with promulgation of the Edict of Milan in 313.

The original Baptistery that was constructed by the Emperor Constantine was a circular building that was inaugurated sometime between 320 and 325AD. Later, Pope Sixtus III (432-440) changed the shape of the Baptistery to its present octagonal layout that we see today.

This baptistry was for many generations the only baptistery in Rome, and its domed octagonal structure, centered upon the large octagonal basin for full immersions, provided a model for others throughout Italy, and even an iconic motif of illuminated manuscripts, "The fountain of Life".

Around the central area, where is the basin of the font, an octagon is formed by eight porphyry columns, with marble Corinthian capitals and entablature of classical form. On the ceiling of the Baptistry is the story of the Battle of the Milvian Bridge (312 AD). An ambulatory surrounds the font and outer walls form a larger octagon. Attached to one side, towards the Lateran Basilica, is a fine porch with two porphyry columns and richly carved capitals, bases and entablatures.

Its plain brick exterior was embellished with a frieze designed by Francesco Borromini in the late 1650s or early 1660s, incorporating the arms of Pope Alexander VII.