The Constantinian baptistery was created by adapting the nymphaeum of an already existing bathing area on the site, but it acquired its present form in the remodelling commissioned by Sixtus III 432-440, the same pope who also built the basilica of Saint Mary Major.
He is responsible for the octagonal plan of the fountain and the double order of columns with the architrave. The octagon symbolically reminds us that time marked out in weeks, as it has been since the first chapter of the Bible, is unfinished, even though Genesis already knows the seventh day as a feast day – it is the great gift of the weekly rest unknown before Judaism, which the Church gave to the whole world.
But that time, which always repeats itself, from week to week, needs a fulfilment, a further day, the eighth, which opens up the flow of time to eternity, through the resurrection of Christ on “the first day after the Sabbath”, as the Gospels remind us.
On the architrave one can read the verses, perhaps of the future Pope Leo the Great, then a deacon, with which Sixtus III wanted the value of the grace of baptism to be announced during the Pelagian dispute (Pelagius believed, unlike the great Church and Augustine who enlightened it on this question, that man could live the faith by his own strength alone, without the need for God’s grace).
The inscription reads thus:
“From this divine seed is born a people to be sanctified, whom the Spirit brings forth from this water, made fruitful. Immerse yourself, sinner, in the holy river, to be purified. The water will restore as new what it has taken in as old. There is no more distance between those who are born again; one Source, one Spirit, one faith unite them. The Mother Church virginally gives birth in this water to the children who were conceived by the inspiration of God. If you wish to be innocent, purify yourself in this washing; whether the paternal guilt [Adam's, that is, original sin] oppresses you or your own. This fountain is life and saves the whole world, originating in the wounds of Christ. Hope for the kingdom of heaven, you who have been born again at this fountain. Those who are born only once do not receive the happy life. Let no one be daunted by the number or form of his sins: he who is born at this river shall be holy”.