The Baptistery of San Giovanni in Fonte is the oldest baptistery in all of Christianity. It is located to the right of the apse of the Santa Restituta Basilica in Naples. Like the Basilica of Santa Restituta, the construction of the Baptistery of San Giovanni in Fonte was initiated by the Roman Emperor Constantine (Flavio Valerio Aurelio Costantino 274 to 337). The construction of the baptistery was directed by the bishops of Naples, although the sources are not entirely unanimous. Bishop Severo (362-408) or Bishop Sotero (465-468) or Bishop Vincenzo (554-578).
The Baptistery of San Giovanni in Fonte is built in a square shape from small blocks of tufa. The top edges are rounded and merge into a dome covered with mosaics from the fifth century. A few fragments of the mosaics can still be seen today. The background dominates in the colors turquoise and green, with parts in gold. The dome of the vault is covered with a starry sky in gold, blue and white with eight rays of different lengths. The monogram of Christ is between the Greek letters Alpha and Omega. Above the cross, the hand of God wears a tied laurel wreath with ribbons fluttering at the ends.
The gold-based frame that surrounds the dome is decorated with branches, trees, fruit baskets on which birds sit. Peacocks, pheasants, partridges and parrots are depicted. A phoenix sits on a small hill between two palm trees. A blue curtain with golden threads and eight garlands of leaves, fruits and birds fall from its border. Four of these garlands with biblical scenes have survived.
Below the dome is the basin for the baptismal rituals.