Thus, the very structure of the place is designed to make visible—and to signify to the eyes and minds of the faithful—the new life in Christ, which is accessed through the sacrament of baptism: "The building with eight niches was raised for sacred purposes / the octagonal font is worthy of this gift. / It was fitting that the hall of holy baptism arose from this number / through which true salvation was restored to the peoples / in the light of the risen Christ [...]” (verses attributed to St. Ambrose).

The water supply system has not survived. If we consider the system devised by Bishop Eustorgius in the Milan Baptistery, where water flowed through pipes placed on the columns, or the Lateran Baptistery, where water gushed from deer-shaped statues, the system in this baptistery must have been equally magnificent.

The long sides of the basin are covered with large marble slabs engraved with jewelled crosses, while the short sides feature geometric patterns. The engravings were filled with coloured plaster, using the champlevé technique, traces of which can still be faintly seen today.