The corners of the octagon are decorated with re-used ancient columns and capitals. The space beneath the cupola is illuminated by a series of large tambour windows, and the walls were probably covered with whitewash rather than mosaics. In the mid-fifth century, baptisteries without mosaic decoration were still being constructed in Italy, the most outstanding example being the Lateran baptistery built by Pope Sixtus III in 432-40. This baptistery preserved eight porphyry columns from its predecessor, the Constantinian baptistery, although these columns themselves had been assembled from more ancient spolia. Sixtus chose an octagonal plan for this structure, whose marble-covered walls must have looked extremely opulent. Its most outstanding preserved features are the capitals and architrave, which selected with great care from classical buildings, the latter bearing an inscription by Pope Sixtus that emphasises the creation of a holy people (gens sacranda) by the rite of baptism and rejects heretical beliefs by saying 'one baptismal font, one spirit and one faith alone'.28

In view of the somewhat competitive relationship between the Lateran church and St Peter's, Sixtus' inscription may have been intended as a challenge issued by the Lateran baptistery to its counterpart. As at St Peter's, mosaics adorn the vestibule apse, which may have been used as a consignato-rium.29 The two shepherds and two sheep symbolise the acceptance ofbelievers into the fold of the Christian congregation. The surviving mosaic is non-figurative. The zenith of the cupola portrays Christ as a lamb with four doves (the evangelists), as well as lilies (symbols of purity and innocence, but also of Christ) and roses. The main motif is an acanthus vine that curls upwards on a blue background and draws the viewer's gaze toward heaven and away from the earthly realm. This blue background is in the tradition of the mosaics decorating nymphaea. The draughtsman of this decorative programme camouflaged his message so that only initiates would understand it.

The mosaic programme at the Baptistery of Naples,30 whose scenes can be 'read' effortlessly, stands in stark contrast to the symbol-laden Lateran decorations. This freestanding baptistery, which was constructed by Bishop Severus (sed. 362-408) within the bishop's palace (intus episcopio), is now located on the right side of the apse of the medieval church of S. Restituta. The structure, which is rectangular both inside and out, has a cupola on squinches. The fact that only the vault and squinches are decorated with mosaics is attributable to a centuries-old practice whereby only domed sections ofpublic

28 ILCV, 1: n. 1513c: 'Unus fons unus spiritus una fides'.

29 I.e., the room for the confirmation ceremony after baptism.

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