The origins of the temple dedicated to St. John the Baptist, later patron saint of the city, are still uncertain. According to tradition, it was founded in Roman times and dedicated to the god Mars. Several sarcophagi have in fact been found in this area, today in the Museum of the Opera del Duomo, as was the famous statue of Mars, which mediaeval chronacles tell us stood at the entrance to Ponte Vecchio. However some scholars think that the building was the Praetorium and the statue that of a barbarian king.
Dante himself declared that his "beautiful San Giovanni" (Inferno, canto XIX) was a classical Roman building; excavations carried out in this century have in fact discovered remains of Roman constructions underneath the Baptistery and the Cathedral, built in the north-eastern area of the first ring of walls.
The foundations of the first Baptistery of San Giovanni, dated from 4th-5th century circa, was certainly built on top of these ancient buildings.
Its octagonal shape, the two lower orders, the attic and the springer of the cupola (in other words its basic architectural structure), date from the early Christian construction, which was possibly altered or completed in the early decades of the 7th century during the Longobard rule.