An in-depth journey on the mysterious origins of the religious and civil heart of medieval Florence. The Baptistery of St John is the religious and civil heart of the ancient Florence and one of the oldest monumental buildings in the city. Its octagonal shape and the presence of the beautiful ancient marble columns arranged along the internal walls made people believe for centuries that it was an ancient Roman temple with a central plan later converted into a church. We see a hypothetical reconstruction of this pagan temple painted by Giorgio Vasari in the Salone dei Cinquecento in Palazzo Vecchio, on the background of the episode of the Foundation of Florence. This legend, reported in the 14th century by Villani in his Chronicles, had had the meaning of celebrating Florence by recalling its noble Roman imperial origins and was strengthened by the discovery of an ancient statuette depicting a character on horseback, which was identified with the God Mars whose temple it would have been dedicated and that it would have been mounted on a column in its center (the statue was then lost in 1333). The remains of Roman Florence were present in the medieval city and around the Baptistery there was a cemetery area where there were late ancient sarcophagi, some of which were reused for monumental burials inside. A marble relief, evidently taken from one of these sarcophagi, and depicting scenes of harvest and maritime trade can be seen walled up outside the apse. But what is the truth? The first historical mention of a place dedicated to St. John the Baptist dates back to 897, while we know that Pope Nicholas II consacrated it in 1059. Furthermore, no clues emerged from the archaeological excavations carried out in the Baptistery area between 1895 and 1948 that have demonstrated the existence of a temple dedicated to Mars. Instead, some water collection and the remains of a floor decorated with elegant mosaics of the house of a rich Roman citizen who lived around the 1st century were found. However the data are still very scarce: how old is the Baptistery? There are four hypotheses ... 1. It was built in the 12th century, later renovated in the apse and then decorated in the 13th century. 2. It was built between the 11th and 12th centuries in place of a baptismal font or small octagonal baptistery much older. 3. It dates back to the Lombard period (7th century ca.) and was later modified and embellished between 1000 and 1299. 4. Its origin dates back to the IV-V century, it was equipped with an octagonal immersion font in the center and only in the early Middle Ages it was renovated and decorated. We only add a consideration on the shape of the octagon which is universal and ancient. Octagonal buildings already existed in ancient Roman times and Saint Ambrose of Milan in the 4th century promoted this shape for the Christian Baptisteries, giving a symbolic meaning to the number of the sides, as a reference to the eighth day of the creation, that is the eternity to which the sacrament of Baptism prepares the soul of the faithful.