In Christian architecture the baptistry or baptistery (Latin baptisterium, from Greek βαπτίζειν) is the separate centrally-planned structure surrounding the baptismal font. The baptistry may be incorporated within the body of a church or cathedral and be provided with an altar as a chapel. In the early Church, the catechumens were instructed and the sacrament of baptism was administered in the baptistery.
The sacramental importance and sometimes architectural splendor of the baptistry reflect the importance of baptism to Christians. The octagonal plan of the Lateran Baptistery, the first structure expressly built as a baptistry, provided a widely-followed model, which might be twelve-sided, or even circular as at Pisa. In a narthex or anteroom the catechumens were instructed and made their confession of faith before baptism. The main interior space centered upon the baptismal font (piscina), in which those to be baptized were immersed thrice. Three steps led down to the floor of the font, and over it might be suspended a gold or silver dove. The iconography of frescos or mosaics on the walls were commonly of the scenes in the life of Saint John the Baptist. The font was at first always of stone, but latterly metals were often used.
The Lateran baptistery's font was fed by a natural spring. When the site had been the palatial dwelling of the Laterani, before Constantine presented it to Bishop Miltiades, the spring formed the water source for the numerous occupants of the domus. It will be quickly apprehended that as the requirements for Christian baptisteries expanded, Christianization of sacred pagan springs presented natural opportunities. Cassiodorus, in a letter written in A.D. 527, described a fair held at a former pagan shrine of Leucothea, in the still culturally Greek region of south Italy, which had been Christianized by converting it to a baptistery (Variae 8.33). In a paper read in 1999, Samuel J. Barnish drew examples of the transition from miraculous springs to baptisteries from Gregory of Tours (died c. 594) and Maximus, bishop of Turin (died c. 466).
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, with infant baptism increasingly the rule, few baptisteries were built. 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 three times in the year.
During the months when there were no baptisms the baptistery doors were sealed with the bishop's seal, a method of controlling the orthodoxy of all baptism in the diocese. 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. The Florentine Antipope John XXIII was buried in the Baptistery facing Florence's Duomo with great ceremony and a tomb erected. Many of the early archbishops of Canterbury were buried in the baptistery at Canterbury.
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. (Main article: Lateran Baptistery.) 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 Saint Peter the New Law charter sealed with the XP monogram.
The earliest surviving structure that was used as a baptistry is the tomb-like baptistry at Dura-Europas . Another baptistery of the earliest times has 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 former patriarchal Church of Holy Wisdom; 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 Baptistère Saint-Jean at Poitiers, and other early examples exist at Riez, Fréjus and Aix-en-Provence. In England, a detached baptistery is known to have been associated with Canterbury Cathedral.
Famous Baptisteries 
Famous Italian baptistries include:
- The Lateran Baptistery, Rome,the most significant and architecturally most influential baptistry in the Christian West, founded by Pope Sixtus III;
- The Baptistry of San Giovanni in Forte, Ravenna;
- The Baptistry of Parma;
- The Tuscan Romanesque Battistero di San Giovanni, associated with Santa Maria del Fiore, the duomo of Florence, rebuilt between 1059 and 1150; it contains Ghiberti's "Doors of Paradise";
- The circular domed Baptistry of St. John clad in white marble in the Piazza del Duomo, Pisa, built in stages from 1150 and combining Romanesque with Gothic.
- The Christian baptistery of Albenga, dating to the 5th century.
- Others are found at Ascoli Piceno, Pistoia and Padua.
Famous French baptistries include:
- The Baptistry of Fréjus Cathedral;
- The Baptistry of Aix Cathedral in Aix-en-Provence;
- The Baptistry of Poitiers, reputedly the oldest Christian building in France.
- The Baptistery of Leurent
Byzantine baptisteries of the Holy Land: Emmaus Nicopolis
See also 
|Wikimedia Commons has media related to: Baptistries|
- Renaissance architecture
- Medieval architecture
- Pieve (church with a baptistery on which other churches without baptisteries depended)
- W. R. Lethaby (1911). "Baptistery". In Chisholm, Hugh. Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Barnish, S. J. B. 2001. "Religio in stagno: Nature, Divinity, and the Christianization of the Countryside in Late Antique Italy" in Journal of Early Christian Studies, vol 9:3, pp. 387–402.