This baptistry of the Lateran was built in the 4th century in Rome. It was one of the first public baptistries built by Emperor Constantine after his conversion to Christianity. Historically, Christian converts were baptized once a year on the eve of Easter. People in droves were baptized by immersion in this octagonal shaped pool. When you look up at the dome inside (which itself is supported by 8 columns), you’ll see an image of the dove of the Spirit in the center. Some baptistries built after this were very large and could hold many baptisms. Before Constantine, baptistries such as this one did not exist. In fact, Christians lived could only live out their faith in secret and were regularly fed to the lions as a game-sport or were burned alive. (Thank God for the working of the Spirit upon political leaders). Before Constantine Christians were likely baptized in secret. It might also be possible that the practice of water baptism was not ritualized like it is today. One thing for sure is that the method of baptism, as a ritual in the post-Constantine period, was done by immersion. It was only in the 6th century that baptism by sprinkling was used with the baptismal font. After the 9th century, infant baptism slowly became popular and fewer baptistries were built.