Although the account of Constantine’s baptism administered by Pope Sylvester is fanciful, it is certain that the emperor personally wanted the monumental baptistery to be built next to the basilica. Constantine was baptized only at the point of death, in the year 337.
The Lateran Baptistery was the first of the city and, like the basilica itself, provided the model for all other ancient baptisteries, at least in the Italian peninsula.
The first Christians of Rome were baptized in the Tiber, as recalled by Tertullian, exactly as in every region of the ancient Church where running and “living” water was used, especially river water.
But soon the community felt the need for places of its own, covered and embellished with Christian images, as community life needed physical places in which to meet and celebrate. In short, the most ancient baptisteries, such as that of Duro Europos in Syria, around the middle of the third century, were born not as a concession to an external power, but rather from the very life of Christians and their need to express their faith in suitable places, with words, signs, hymns and appropriate artistic expression.