The Diocese of Castello, originally the Diocese of Olivolo, is a former Roman Catholic diocese that was based on the city of Venice in Italy. It was established in 774, covering the islands that are now occupied by Venice. Throughout its existence there was tension between the diocese, the Patriarchate of Grado to which it was nominally subordinate, and the Doge of Venice. Eventually in 1451 the diocese and the patriarchate were merged to form the Archdiocese of Venice.
History
Foundation
The diocese has its origins in the Patriarchate of Aquileia, founded during the Roman Empire. Hilarius of Panonia is recorded as bishop of Aquileia from about 276–285. As the empire fell into decline, Aquileia was sacked in turn by Visigoths (403), Huns (452) and Lombards (659). During these times of trouble some of the people would take refuge on the offshore islands. By 630 an independent Patriarch of Grado was established on the island of Grado.
The islands of Venice had originally been subject to the Diocese of Padua. In 774 Pope Adrian I and John IV, Patriarch of Grado, authorized the establishment of an episcopal see on the island of Olivolo; the island is now called San Pietro di Castello. The bishopric was established in 774–775 by the Duke of Malamocco, who gave it his protection. Its cathedral was dedicated to Saint Peter. The Bishop of Olivolo was subordinate to Grado and had jurisdiction over the islands of Gemini, Rialto, Luprio and Dorsoduro, the main islands of the city of Venice. The bishopric, taken from the Diocese of Malamocco (Methamancus), formed a small new state, the nucleus of the state of Venice.
Olivolo
The first bishop was Obelerius. He was invested and enthroned by the doge, and consecrated by the Patriarch of Grado. In 798 the doge nominated Cristoforo as his successor. Giovanni, patriarch of Grado, refused to consecrate Cristoforo due to his youth. Giovanni was killed for his disobedience and his successor consecrated Cristoforo. From his name, which is Greek, Cristoforo may have been Byzantine.
At that time Italy was the seen of a struggle between the Lombards and Byzantines, and many in Venice were loyal to Byzantium. The Franks rose to power in the last half of the 8th century, and in 800 Pope Leo III crowned Charlemagne as Emperor. In 802 a faction that was friendly to the Franks came to power in Malamocco, and sent the Doges Giovanni and Maurizio II into exile, along with the bishop of Olivolo. In 810 a Byzantine fleet helped restore the party that favored Byzantium, and Angelo I Participazzo was made doge. An attempt by the Franks to conquer the Venetian Lagoon failed, and after long negotiations it was agreed that Venice fell in the Byzantine sphere, while Venetian merchants could trade throughout the western empire. The Doge made his seat on the island of Rialto, and with the surrounding islands including Olivolo the new state started to take the name of Venetiae.
In the ninth century the relics of saints Sergios and Bacchus were placed in the cathedral at Olivolo by the pro-Greek faction. The church of San Pietro di Castello was built by Orso Participazio, the fourth bishop of Olivolo. It was burned down several times.
In 828, the second year of the Doge Giustiniano Participazio, the Caliph had ordered that the Christian churches of Alexandria, Egypt, be pulled down so their marble columns could be used for his palace. The body of Saint Mark the Evangelist was smuggled out from the church of Saint Mark to Venice. To discourage the "Saracens" from examining the coffin, it was filled with pork. When the ship reached Olivolo island in Venice, the saint made signs that showed he did not want to be placed in the custody of the bishop. Instead, he was taken to the Doge's chapel, and planning began to create a magnificent new temple suitable for such important relics. The motives for the robbery of the saint's body were in part to establish the importance of the city of Venice, as opposed to the sees of Grado and Olivolo, relative to the patriarchate of Aquileia.