This year, Christians around the world commemorate the 1,700th anniversary of the First Council of Nicaea, a foundational moment in Church history and a catalyst for a centuries-long conversation about Christian unity — what today we call ecumenism.
The Council of Nicaea allows us to "see in a different light the points that still separate us," Pope Leo XIV told participants of a Catholic-Orthodox symposium during an audience at the Vatican on June 7, 2025.
Convened in 325 by Emperor Constantine in the city of Nicaea (modern-day İznik, Turkey), the Council sought to resolve doctrinal disputes threatening the coherence of the Church in a rapidly changing empire.
Most famously, it confronted the teachings of Arius, who denied the full divinity of Christ, and produced what we now know as the Nicene Creed.
But beyond theological declarations, the Council represented the first time bishops from across the known Christian world gathered to settle matters of faith — making it, in historical terms, the first “ecumenical” council.
Seventeen centuries later, the word ecumenical has taken on new meaning. Originally describing a council of united believers, it now refers to the effort to build unity among Christians who remain divided — across Catholic, Orthodox, and Protestant lines. This shift reflects a painful truth: While the early Church sought unity, it has long lived with division.
Pope Leo XIV told his guests that he was "convinced" that by returning to the Council of Nicaea, "a common source" for the Catholic, Orthodox, and Eastern faiths, it would be possible to see that division in a different light. The anniversary could help us better understand "the mystery that unites us" and move toward "the restoration of full communion," he proposed.
So what is ecumenism?
Put simply, ecumenism is the movement among Christians to overcome divisions and seek visible unity. It’s rooted in Christ’s prayer “that they may all be one” (John 17:21). For the Catholic Church, the Second Vatican Council marked a turning point. In 1964, Unitatis redintegratio (“Restoration of Unity”) recognized division among Christians not just as a challenge, but as a scandal—one that harms the credibility of the Gospel.
Fr. Elias Mallon, writing for CNEWA, explains that the idea of Christian unity has evolved. While theological dialogue remains essential, relationships now often begin with friendship and cooperation. Perfect unity, he notes, may be an eschatological goal, but mutual love and collaboration are possible now.
The 1,700th anniversary of Nicaea is not just a chance to recall ancient debates. It’s a living reminder of the Church’s ongoing responsibility to pursue unity.
Late Pope Francis recognized this in 2024 when he proposed a joint observance of the anniversary with the Ecumenical Patriarchate in Constantinople, calling it an opportunity “to bear witness to the growing communion that already exists among all who are baptized.”
A real communion already exists
That communion, while incomplete, is real. The annual exchange of delegations between Rome and Constantinople — on the feasts of Saints Peter and Paul, and of Saint Andrew — signals a shared recognition of common faith, heritage, and mission.
And it’s not only about dialogue at the top. Ecumenism touches the daily lives of ordinary Christians — through shared prayer, cooperation in humanitarian and pro-life efforts, and a growing understanding that unity isn’t about erasing differences but sanctifying them in truth and love.
As the Church celebrates Nicaea’s 1,700th anniversary, the question isn’t only what the Council accomplished in the past. It’s what it still calls us to do now: to seek unity not as an ideal, but as a path of fidelity to the Gospel, and a witness to the world.
Easter
The Pope also addressed the issue of the date of Easter, which Catholics and Orthodox Christians calculate differently, but which this year was celebrated on the same day due to the coincidence of the Julian and Gregorian calendars.
"I reaffirm the Catholic Church's openness to seeking an ecumenical solution that promotes a common celebration of the Lord's Resurrection," he said, as his predecessor had also said repeatedly before him.
