This very early baptistery, reconstructed in the 5th century, played a significant role in the development of baptistery architecture, as it was the only such structure in Rome for many decades. The octagonal, windowed dome echoed the baptismal basin directly below, setting a pattern for future structures, as Christianity spread.

"The octagonal baptistery adjoining the Lateran basilica constructed under the auspices of Constantine had in its second phase a circular baptismal font with a depth of 91 cm below floor level. This Lateran baptistery was reconstructed under Sixtus III (432-440), and the font was given its present octagonal shape. The baptismal pool was .5 meters in diameter, and so designed for multiple immersions. Sixtus placed a Latin inscription over the eight sides of the entablature of the colonnade surrounding the room. It sets forth his baptismal theology:

"Here is born from life-giving seed a people, consecrated to another city, whom the Spirit brings forth from the fertile waters.To plunge in the holy purifying flood the sinner, whom the wave receives as old but gives forth as hew. None reborn is different from those it makes one, One font, one Spirit, one faith. Mother church as a virgin brought forth those who are born, whom she conceived by the divine breath and brought into being in the flowing water.The person who wants to be innocent is here made clean by washing, whether from the guilt of the first parent or one's own.Here is the font of life which bathes the whole world, its ultimate source the side of Christ wounded. Reborn in this font for the kingdom of heaven, the blessed life does not receive those born only once.Be no afraid of the number or kind of your sins, for the one born in this river will be holy." (Ferguson, p. 769, and from Brandenburg, Ancient Churches of Rome, p.324.)