On the feast of Christ the King, Pope Leo XIV published the apostolic letter In unitate fidei, written for the 1700th anniversary of the Council of Nicaea. In it, he invites the entire Church to rediscover the unifying force of the Niceno-Constantinopolitan Creed - a profession of faith that for centuries has created common ground among Christians of every tradition.

The Pope begins by recalling the central message of the council held in 325: the confession of Jesus Christ as the “only-begotten Son of God,” the heart and foundation of Christian faith. From this truth, he says, flows the responsibility to guard and proclaim the Gospel in an age marked by war, violence and deep social and environmental turmoil.

Drawing on a recent document of the International Theological Commission on the continuing relevance of Nicaea, Leo XIV proposes an honest spiritual examination. What place does God hold in our daily lives? Which “idols” end up taking precedence over His commandments? And how truly do believers care for creation - a gift entrusted to all?

Alongside personal reflection, the Pope stresses the urgency of unity. The Creed formulated at Nicaea, he notes, is still professed in Christian liturgies around the world and remains the theological cornerstone of ecumenical dialogue. While acknowledging the distance still to be travelled before full communion, Leo XIV highlights the real progress made in recent decades and encourages Christians to move beyond long-settled disputes, opening themselves to a “reconciliation that looks to the future.”

Unity, he insists, is not sameness but “harmony within diversity.” His warning is clear: “unity without plurality becomes tyranny; plurality without unity leads to fragmentation.” For this reason, he calls for a renewed spiritual ecumenism grounded in listening, prayer, mutual hospitality and the desire to walk together as disciples of Christ.

The Pope also reflects on the historical context of Nicaea, born in a period of fierce theological conflict. Yet from that crisis emerged a confession of faith strong enough to guide the Church for centuries. Today the challenge is different: what divides Christians is not internal debate so much as the growing marginalization of God in contemporary life. Leo XIV acknowledges that Christians themselves share responsibility whenever their lives fail to reflect the authentic face of the Gospel.

He therefore urges believers to return to the heart of Christian faith: to follow Jesus with sincerity, recognize the dignity of the vulnerable and draw close to those who suffer. Only through such witness, he concludes, can men and women of our time perceive the mercy of God.