The Arian Baptistery is an important example of the Gothic domination and of their worship; it is believed in fact built by Theodoric during his rule (493-526) as a baptistery of ancient Germanic Cathedral, today the church of the Holy Spirit, which is located in its vicinity. After the expulsion of the Ostrogoths from Ravenna and after the Byzantine conquest (540), was issued by the emperor Justinian issued an edict by which the sacred buildings Aryans were reassigned to the Catholic Church; therefore the building was reconciled to the Orthodox worship at the time of the archbishop Agnello as a chapel dedicated to the Virgin Mary. Hence the name of Santa Maria in Cosmedin with which was identified from the seventh century, and the name of Monasterium Sanctae Mariae with which remembers the proto Ravenna Andrea Lamb in the ninth century. The Baptistery, whose stories were related in part to those of the adjacent cathedral Aryan, then underwent many changes, including the demolition of the apses and the raising of the floor. In the eleventh century it was taken over, along with the nearby Church of the Holy Spirit, by the Benedictines; in the Renaissance it was in commendam to secular clergy and from 1608 he was entrusted to the care the Teatini. Particularly important were the interventions that interested him in the seventeenth century; In fact, since 1667, through the intervention of Cardinal Cesare Rasponi, it was incorporated to an adjacent building where he function apse and formed along with it the Oratory of the Brotherhood of the Cross. At the end of the nineteenth century the building was privately owned, he had lost all religious function and was surrounded by accretions. In 1914 he became the property of the Italian State, and between 1916 and 1919 was affected by major restoration work which provided important information on set original architecture, as a result of which reestablished whenever possible the parties demolished and brought out all that It remained of the ancient buildings.
The building, which now presents itself partly buried, is structured with an octagonal central plan. Protrude from the perimeter four small semicircular apse oriented to the cardinal points, of which the largest, facing east, is preceded by a presbytery. What we see today is only the central part of the old building, which appeared fuller and more numerous; In fact originally the baptistery was surrounded by an annular ambulatory covered by a vault, which was interrupted only on the eastern side, in larger apse correspondence. They are still visible at the smaller apses the arches that served the junction of the time. Inside the walls are bare brick, but in ancient were richly decorated with marble, stucco and mosaics. Of all the original decorative scheme is needed now is the precious mosaic decoration of the dome, in which scholars, while the unity of the iconographic program, found different stylistic times; the currently prevailing opinion is however that all operations date back to teodericiano period. In addition, although the mosaic substantially well preserved, the inevitable restoration that the representation has undergone over the centuries explain some inconsistency and the difficulty of interpretation of some details. The iconographic setting mosaic denounces the influence of the figurative program implemented, a few decades earlier, within the adjoining Baptistery of the Catholic cathedral, called Neoniano; but it differs because of the changed artistic taste for the different influence of philosophical trends, especially the rise of the Arian doctrine. This heresy that stood out above all the affirmation of the human nature of Christ and the denial of coexistence with the divine, found widespread use among the Germanic tribes and established himself in Italy precisely with domination of Theodoric. At the center of the dome stands a medallion framed by a ring with golden laurel wreath on a red background; inside it is depicted, as is appropriate in a baptistery, the scene of the baptism of Christ, symbol of victory over death. The episode is traditionally constructed with three characters. At the center of the naked Christ, immersed in the waters of the Jordan, it is presented as young and beardless, while the Baptist left, dressed in a rough habit and a shepherd’s crook symbol of the life of the desert hardships, imposes his hand on the head in accordance with the ancient ritual. Vertically down from the divine dove, to spray with a puff of light, symbol of the Spirit, the head of Christ (according to other interpretations, with a stream of holy water, or the bird holding in its beak an olive branch , alluding to the return of peace after the flood). On the right there is the figure of a noble and powerful old man with white hair and beard, represented with the naked torso and lower body covered with a green cloth. The character symbolizes the Jordan River, as provided by attributes that are derived directly from common personified river gods Hellenistic iconography. In fact the figure leans on an inverted pot, from whose mouth flows the water, and holding in his hand a reed, and on his head sprouting red claws of a crab, that represent elements of aquatic life. In concentric layer that surrounds the medallion depicts a majestic procession of apostles, who advance with a lilting rhythm on a gold background, interspersed with stylized images of palm trees. Dressed according to the style of the ancient Romans, each supporting a crown, symbol of victory, his hands covered, out of respect, by a white cloth called the pallium. On each of these veils appear Greek letters such gammadie, these monograms arising from classical pagan world which then acquired an important meaning in the context of the Christological symbols and numerology, but today we can not interpret it with ease. The procession ended at the right from the left by Peter and Paul, well characterized in the faces, and identified by the canonical attributes: one is holding the keys and the other two rolls. They flanked the symbolic culmination of the procession: a golden throne richly decorated. The Christian iconographic motif of the empty chair, oriental and Etimasia called, refers to the invisible presence of Christ and symbolically represents the throne on which he will sit in the day of final judgment. Here, however, the iconography is enriched by new elements that change the interpretation and that might be the contribution of the Aryan doctrine of the depiction. In fact, the seat is placed a white cloth and a purple pillow where rests a large Latin cross adorned with jewels; these items may be symbolic of the suffering on the cross and then to allude to human nature and the physicality of Christ. Since 1996, the Arian Baptistery is one of the monuments protected by UNESCO.