The baptistery of San Giovanni in Fonte or baptistery of Marcelliano is one of the oldest Christian baptisteries in all of the West. It was named in honor of Pope Marcellus who, during his brief pontificate (308-309), resumed the difficult program of giving an organic religious arrangement to the territory, interrupted by the ferocious persecution of Diocletian. Pope Marcellus as part of an extension of the organization of the Catholic Church established new dioceses, appointed other bishops and favored the construction of a baptistery for each diocese.

The unicum of this place is represented by the fact that the baptismal pool, instead of being filled artificially as usually happened in other buildings, received water in a natural way because it was built on a perennial spring, allowing baptism by immersion. This makes the Early Christian Baptistery of San Giovanni di Marcellianum unique in the world of Christianity.

The original building has a square plan with brick arches and corresponds to the room in which the large baptismal font is located flanked by two ambulatories, while the other structures, such as the chapel and the portico, date back to a later period. In the chapel there were the frescoes of the 4 evangelists translated and kept in the Certosa di S. Lorenzo.