Baptistery (Baptisterium, in the Greek Church φωτιστήριον), the separate hall or chapel, connected with the early Christian Church, in which the catechumens were instructed and the sacrament of baptism administered. The name baptistery is also given to a kind of chapel in a large church, which serves the same purpose. The baptistery proper was commonly a circular building, although sometimes it had eight and sometimes twelve sides, and consisted of an ante-room (προαύλιος οἶκος) where the catechumens were instructed, and where before baptism they made their confession of faith, and an inner apartment where the sacrament was administered. In the inner apartment the principal object was the baptismal font (κολυμβήθρα, or piscina), in which those to be baptized were immersed thrice. Three steps led down to the floor of the font, and over it was suspended a gold or silver dove; while on the walls were commonly pictures of the scenes in the life of John the Baptist. The font was at first always of stone, but latterly metals were often used.
Baptisteries belong to a period of the church when great numbers of adult catechumens were baptized, and when immersion was the rule. We find little or no trace of them before Constantine made Christianity the state religion, i.e. before the 4th century; and as early as the 6th century the baptismal font was built in the porch of the church and then in the church itself. 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. (See Baptism.) During the months when there were no baptisms the baptistery doors were sealed with the bishop's seal. Some baptisteries were divided into two parts to separate the sexes; sometimes the church had two baptisteries, one for each sex. A fireplace was often provided to warm the neophytes after immersion.
Though baptisteries were forbidden to be used as burial-places by the council of Auxerre (578) they were not uncommonly used as such. Many of the early archbishops of Canterbury were buried in the baptistery there. 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. They are still in general use, however, in Florence and Pisa. The baptistery of the Lateran must be the earliest ecclesiastical building still in use. A large part of it remains as built by Constantine. The central area, where is the basin of the font, is an octagon around which stand eight porphyry columns, with marble capitals and entablature of classical form; outside these are an ambulatory and outer walls forming a larger octagon. Attached to one side, towards the Lateran basilica, is a fine porch with two noble porphyry columns and richly carved capitals, bases and entablatures.
The circular church of Santa Costanza, also of the 4th century, served as a baptistery and contained the tomb of the daughter of Constantine. This is a remarkably perfect structure with a central dome, columns and mosaics of classical fashion. Two side niches contain the earliest known mosaics of distinctively Christian subjects. In one is represented Moses receiving the Old Law, in the other Christ delivers to St Peter the New Law - a charter sealed with the X P monogram.
Another baptistery of the earliest times has recently been excavated at Aquileia. Ruins of an early baptistery have also been found at Salona. 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. To the latter date also belongs a large baptistery decorated with mosaics at Naples.
In the East the metropolitan baptistery at Constantinople still stands at the side of the mosque which was once the patriarchal church of St Sophia; and many others, in Syria, have been made known to us by recent researches, as also have some belonging to the churches of North Africa. In France the most famous early baptistery is St Jean at Poitiers, and other early examples exist at Riez, Fréjus and Aix. In England, a detached baptistery is known to have been associated with the cathedral of Canterbury.
See Hefele's Concilien, passim; Du Cange, Glossary, article "Baptisterium"; Eusebius, Hist. Eccl. x. 4; Bingham's Antiquities of the Christian Church, book xi.
(W. R. L.)