The Baptistery, probably an old Roman bath (nymphaeum), whose original floor, three meters below the present level, was transformed into the Baptistery by Bishop Ursus (early fifth century, and decorated with mosaics under Bishop Neon after the year 452, so that it is also known as the Neonian baptistery.
The actual floor is the fourth.
Similarly to the Gall Placidia mausoleum mosaics, the mosaics of the baptistery bear the influence of the roman classic art.
In the dome, which is also built of hollow terracotta pipes, there are three groups:
In the center, in a disc, the Baptism of Christ. The Saviour stands half-immersed in the water of the Jordan; to His right is shown John the Baptist pouring the water on His head, while to His left, there emerges from the water the nude bearded figure of an elderly man; the personification of the Jordan river.
Then the twelve Apostles. A name at the side of the head countermarks each Apostle.
Below: eight small temples with four altars – each with the gospel book opened on it – alternate with four thrones bearing a cross the so called etimasia or symbolic preparation of the throne of God for the last Judgment.
Under the eight large arches of the windows, small stucco shrines of the prophets.
The baptismal font in center, has been remodeled according to its original shape and style which used for baptism by immersion. Of the font there only remains the small pulpit in Greek marble.
There is also a marble vase, which probably was used for the purification in pagan nuptials.
Also a small antique altar.