The Arian Baptistery, built in an octagonal shape, is a remnant of the period when Arianism was still recognized by the Catholic Church. After the doctrine was declared heretical, the church was converted to Catholic use. For centuries, the baptistery remained private property until it was transferred to the Italian government in 1914.

During World War II, the baptistery was bombed, and among the ruins, additional walls belonging to the original structure were uncovered.

It’s especially interesting to observe the 50-year gap in construction between this baptistery and the Neonian Baptistery, particularly in the different depictions of Christ in the central mosaic: here, Jesus is shown young and beardless, in contrast to the older, bearded figure seen in the other baptistery.