The Baptistery of St. Giovanni in Fonte next to the Church of St. Restituta is considered the most ancient Baptistery of the western Mediterranean.
The building, probably founded in the second half of the fourth century by Severo, Bishop of Naples, is characterized by a squared-plan whose edges create an octagonal drum culminating in a dome. This unique structure has oriental architectural elements and wonderful mosaics.
They are among the most precious ones, certainly more ancient than the ones in Rome and Ravenna.
Under the dome there is a baptismal font, dug out of the floor, where the Catechumen knelt and were baptized with ablutions of water. The newly baptized could have risen up their faces and admire the extraordinary mosaics which introduced them to a mystic and holy world.