The Interior
The truly outstanding feature of the interior is the mosaic dome ceiling, begun in 1225 and completed around 100 years later.  We will look at it below, here, but first let's look at the other internal features.   Originally, as with other baptisteries, the font was very large, designed for total immersion. There is a legend that Dante rescued a child drowning in it.  It was replaced by the present small font in 1571.    A perhaps unlikely feature is the grand tomb of Antipope John XXIII, created mainly by Donatello and Michelozzo Michelozzi. So why is this here? The answer is that he had plenty of money: he donated some of it to the building of the baptistery. He also supplied it with relics.
The present font	Tomb of Antipope John XXIII
On to the mosaic ceiling, one of Italy's greatest works of medieval art. Impossible to do it justice here, so think of this as an introduction!
The octagonal ceiling is divided into narrative sections around the central lantern. This diagram (taken from elsewhere) explains the plan.
Section 1 shows the Last Judgement. Section 2: the Heirarchy of Angels. Section 3: scenes from Genesis. Section 4: the life of Joseph. Section 5: the birth of Christ and scenes from the Passion. Section 6: the life of John the Baptist. Here is section 1, the Last Judgement, and a close up of the scene from Hell. To the side of Christ are three registers: the top one shows angels, the central one the Virgin, John the Baptist and  the Twelve Apostles, and the lower register Paradise and Hell.
Here is a selection of mosaics from the various sections of the roof. First, Thrones from the hierarchy of angels. The depiction of the complete hierarchy is unique in art. A problem for the mosaicists was that the arrangement of the dome presented them with eight sections, not the required nine: the solution was to put the most important of the angels, the Seraphim and Cherubim, together, next to the image of Christ. For more on the angelic hierarchy and images from the Florence batistery visit my posting on angels in art here.
From the Genesis mosaics: the temptation of Adam.
From the Life of Joseph: Joseph interprets the dream of the Pharoah.
From the life of Christ: the Last Supper.
From the life of John the Baptist: the Baptism of Christ.
Finally, below the image of Christ is the entrance to the apse. The celing here has images of Christ, the Virgin, apostles and prophets.
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