The Council of Nicaea 1,700 years ago recognized that Christian unity had to be based on a common faith and should be demonstrated by a common celebration of Easter, the most sacred feast of the Christian year, said speakers at a Rome conference.

Yet as Christians mark the anniversary of the council, held in 325, they celebrate their common profession of the basics of faith in the Creed adopted at Nicaea while also continuing to experience division, said Paul L. Gavrilyuk, president of the International Orthodox Theological Association.

The association and the Institute for Ecumenical Studies of Rome's Pontifical University of St. Thomas Aquinas organized the June 4-7 conference with the support of the Vatican Dicastery for Promoting Christian Unity.

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Gavrilyuk, who holds the Aquinas Chair in Theology and Philosophy at the University of St. Thomas in St. Paul, Minnesota, and the other speakers noted the coincidence of the anniversary year and the election of Pope Leo XIV whose episcopal motto is "In Illo uno unum," an expression of St. Augustine meaning "In the One (Christ), we are one."

"Nicaea was a landmark exercise in collective truth seeking and discernment with an enduring and universally significant dogmatic outcome enshrined in its famous creed," Gavrilyuk said.

The fact that Christians today continue to use the Nicene-Constantinopolitan Creed, not just liturgically but as a statement of orthodox Christian belief, means it is a continuing source of Christian unity, speakers said.

"The restoration of the unity of the Church requires agreement on the essential content of the Christian faith, not only among the churches and ecclesial communities of today, but also in continuity with the Church of tradition, and above all, with its apostolic origins," said Cardinal Kurt Koch, prefect of the Dicastery for Promoting Christian Unity.

The Creed is "the strongest ecumenical bond of the Christian faith," the cardinal said. "The Council of Nicaea took place at a time when Christianity had not yet been divided by so many subsequent schisms; its creed is therefore shared by all Christian churches and ecclesial communities, uniting them in a common confession in this day. Its ecumenical importance cannot be underestimated."

Orthodox Metropolitan Job of Pisidia, a theologian and Orthodox co-chair of the Catholic-Orthodox theological dialogue, said the Council of Nicaea could serve as a model of Christian unity today because it determined the essential points of Christian faith while allowing differences on other matters.

"The Nicene Creed does not represent a confession of faith at a particular moment in the history of the Church but rather manifests the confession of faith that transcends the limits of time and space," he said. It was introduced into the liturgy at the turn of the sixth century, "which shows how much this text became a universal confession of the faith confessed by the one Church, received from Christ through the apostles and handed down by the holy fathers."

Koch said the celebration of the Nicaea anniversary also is an occasion to make a renewed commitment to synodality — shared listening, reflection and discernment — and for members of different churches to learn from the synodal structures of each other's churches.

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"The creed of the Council of Nicaea is not merely the result of theological reflection, but the expression of a joint, more precisely, synodal struggle of bishops for an orthodox and doxologically appropriate formulation of the Christian faith," Koch said. At the time of Nicaea, there were about 1,800 Christian bishops, and most experts believe about 318 of them participated in the council.

The Council of Nicaea also is known for setting the formula for determining the date of Easter at a time when Christian communities were celebrating Jesus' resurrection on different dates. A common celebration of Easter held until Pope Gregory XIII reformed the calendar in 1582.

Koch reminded his audience that since the 1960s, the Catholic Church has repeatedly said it would accept an ecumenical proposal for returning to a common date for Easter "on the condition that all Christian churches reach an agreement."

"The endeavour to find a common date for Easter is an important pastoral concern, particularly for families of different denominations, and in light of the increasing mobility of people today," the cardinal said. "Above all, a shared celebration of Easter would bear more credible witness to the profound conviction of the Christian faith that Easter is not only the oldest but also the central and most important feast of Christianity."

At Nicaea, Job said, the bishops determined that the Church would celebrate Easter on the first Sunday after the full moon after the spring equinox — a formula based on "observable astronomical phenomena" and not on any specific calendar.

"All Christians today, without exception, determine the date of Easter according to the Nicene rule," he said, but with Western Christians using the more accurate Gregorian calendar and Eastern Christians using the Julian calendar, the celebrations only occasionally coincide.

A decision on a proposal for the Orthodox churches "to use the most accurate scientific data to determine the date of Easter, using as a reference the meridian of Jerusalem, place of death and resurrection of Christ," has been postponed multiple times, he said.

Anglican Archbishop Rowan Williams, the former Archbishop of Canterbury, looked specifically at how celebrating the Nicaean anniversary is a call to deeper faith and to greater unity.

To profess the Creed is to profess belief in the Trinity, a community of life that gives life to the Church, the body of Christ, the archbishop said.

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"The unity of the Church is neither a goal toward which human negotiators struggle, nor a timelessly given identity untouched by history," he said. Rather, "it is a constantly realized and constantly frustrated or denied movement between subjects, bringing one another alive in the one life of the eternal Son.

"The faith articulated at Nicaea and later in Constantinople cannot, I would say, be understood just as a set of claims about the life of God in abstraction from the call of God into the life of the new creation," he said.