One of the council's lasting legacies is the Nicene Creed, a profession of faith still recited by Christians of all big denominations.
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Ecumenical Patriarch Bartholomew I led a liturgy in Istanbul on Sunday, marking the 1,700th anniversary of Christianity's first ecumenical council.
The first Council of Nicaea (located in modern-day Turkey, in the ciy of İznik) took place in 325 AD and the date remains important to Orthodox-Catholic relations.
It saw Roman Emperor Constantine I bring together some 300 bishops, according to the Catholic Almanac.
Among the outcomes was the Nicene Creed, a profession of faith that is still recited by Christians today and required of those undertaking important functions within the Orthodox, as well as the Catholic and Lutheran Churches.
As Ecumenical Patriarch of Constantinople, Bartholomew I is "primus inter pares" (first among equals) among the patriarchs of the Eastern Orthodox Church and is its spiritual leader.
Bartholomew, who is known for his ecumenical as well as humanitarian and ecological efforts, had met with Pope Leo XIV earlier this week for the first time.