We have already spoken of the eucharistic celebrations of which the cubicula were the scene; and still existing baptisteries prove that the other sacrament was also administered there.
The font itself is interesting for its early form, one common in the chief baptisteries of northern Italy: like an island in the centre of the great octagonal tank is a lobed marble receptacle, in which the officiating priest stood while he immersed the catechumens.
Baptisteries belong to a period of the church when great numbers of adult catechumens were baptized, and when immersion was the rule.
After the 9th century few baptisteries were built, the most noteworthy of later date being those at Pisa, Florence, Padua, Lucca and Parma.
Some of the older baptisteries were very large, so large that we hear of councils and synods being held in them.
It was necessary to make them large, because in the early Church it was customary for the bishop to baptize all the catechumens in his diocese (and so baptisteries are commonly found attached to the cathedral and not to the parish churches), and also because the rite was performed only thrice in the year.
Some baptisteries were divided into two parts to separate the sexes; sometimes the church had two baptisteries, one for each sex.
Though baptisteries were forbidden to be used as burial-places by the council of Auxerre (578) they were not uncommonly used as such.
Baptisteries, we find from the records of early councils, were first built and used to correct the evils arising from the practice of private baptism.
As soon as Christianity made such progress that baptism became the rule, and as soon as immersion gave place to sprinkling, the ancient baptisteries were no longer necessary.
At Ravenna exist two famous baptisteries encrusted with fine mosaics, one of them built in the middle of the 5th century, and the other in the 6th.