The building of the Bishopric of Nikaia is located in the square of Osias Xeni. It houses the ecclesiastical museum, which was inaugurated in December 1972, 50 years after the Asia Minor Catastrophe. The soul and creator of the church museum was the Metropolitan of Nikaia. Many icons and objects of great religious value and art are preserved in the museum. Among them are the Pantocrator with the characteristic Asia Minor gold treatment, the Epitaph Cububicle and the gold-embroidered Epitaph of Osia Xeni (donated by the Association of the Community of Nisi Eirdir of Pisidia), six-winged carvings, woodcarvings, a bread basket, a Gospel, old books, relics of liturgical use, documents and personal items. There is preserved a mosaic composition of mosaics collected from the excavations in the ruins of the Church of the Assumption of the Virgin of Nicaea Bithynia, where the First Ecumenical Council was held.

The visitor has the opportunity to admire post-Byzantine icons, mainly of the 18th and 19th centuries and some of Russian origin. Some of these icons were brought by the refugees themselves and others come from old churches in the area. However, most of them were transferred to Greece in 1924 through an exchange, following the actions of Metropolitan Yervasios of Grevena, who saved a large number of religious relics.

info. Evangelia I. Hadjifoti, “The Ecclesiastical Museum of the I. Metropolis of Nikaia”, in Holy Metropolis of Nikaia, Nikia History - Theology - Culture 325 - 1987, Nikaia 1988, pp. 339-351.