The sacramental importance and sometimes architectural splendor of the baptistery reflect the historical importance of baptism to Christians. The octagonal plan of the Lateran Baptistery, the first structure expressly built as a baptistery, provided a widely followed model. The baptistery 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 thrice immersed. 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. As the requirements for Christian baptisteries expanded, Christianization of sacred pagan springs presented natural opportunities. Cassiodorus, in a letter written in AD 527, described a fair held at a former pagan shrine of Leucothea, in the still culturally Greek region of south Italy. This shrine had been Christianized by converting it to a baptistery (Variae 8.33). There are also examples of the transition from miraculous springs to baptisteries from Gregory of Tours (died c. 594) and Maximus, bishop of Turin (died c. 466).