The Greek Byzantine Catholic Church occupies a distinctive theological position: it follows the Byzantine rite — the liturgical tradition of the Eastern Orthodox Church — while remaining in full communion with Rome and the Pope. This combination, sometimes called 'Eastern Catholic,' emerged historically from communities that maintained Greek Orthodox forms of worship but entered into union with Rome at various points across the centuries. The Holy Trinity Cathedral in Beyoğlu is the seat of the Greek Apostolic Exarchate of Istanbul, known formally by its Latin title, Exarchatus Apostolicus Constantinopolitanus. Created in 1911 from parts of Ottoman territory, the exarchate was from its founding a small and vulnerable institution in a city where the Greek Orthodox Ecumenical Patriarchate, located just a few kilometers away in the Fener district, represents the far larger Greek Christian presence.

The cathedral's neighborhood tells its own story. Beyoğlu — the district across the Golden Horn from the old historic peninsula, connected to it by the Galata Bridge — has long been Istanbul's most cosmopolitan quarter. For centuries it was home to European merchants, Levantine traders, Armenians, Greeks, and Jews. Embassies, consulates, churches of half a dozen denominations, and the nineteenth-century covered arcade of the Çiçek Pasajı all crowd together here. Hamalbaşı Caddesi, where the cathedral stands, is a few minutes' walk from the famous pedestrian boulevard of İstiklal Avenue. In this context, a Greek Catholic cathedral occupying a small but permanent address is less surprising than it might seem elsewhere — Beyoğlu has always accommodated minority faith communities, even as the communities themselves have changed.

Istanbul is home to three Catholic cathedrals, each representing a distinct tradition. The Cathedral of the Holy Spirit on Cumhuriyet Caddesi in Harbiye represents the Latin rite — the tradition of Rome's own liturgical practice, used by the majority of Catholics worldwide. St. Mary of Sakızağaç Cathedral serves the Armenian Catholic community, one of the city's longest-established minority groups. And the Holy Trinity serves, or has served, the Greek Byzantine Catholics. The coexistence of these three cathedrals, each rooted in a different ancient tradition and now ministering to communities that are shadows of their historical size, is itself a kind of monument to the complexity of Istanbul's religious history — and to the persistence of communities that refuse to entirely disappear.

The cathedral's current role as a worship space for Chaldean Catholic refugees from the Eparchy of Diyarbekir adds a further layer of meaning to an already layered place. The Chaldean Catholic Church is an Eastern Catholic Church of the Chaldean rite — yet another of the distinct liturgical traditions that make up Catholic Christianity — with historic roots in present-day Iraq and southeastern Turkey. Displacement and exodus have characterized this community's modern history. That they have found a temporary home in a Greek Catholic cathedral, whose own founding community has dwindled to sixteen, is less an administrative accident than a reflection of Istanbul's long experience as a city of welcome for those who have nowhere else to go. Two small, displaced communities share a space neither built for the other. The doors of Holy Trinity remain open.