On Sunday, the Solemnity of Christ the King, Pope Leo XIV released a sweeping Apostolic Letter, In Unitate Fidei, commemorating the 1,700th anniversary of the Council of Nicaea and urging Christians everywhere to defend the faith “handed down since the beginning” with conviction; but to do so in a spirit shaped by charity, humility, and love.
The letter, published ahead of the Pope’s Apostolic Journey to Turkey, situates today’s challenges alongside the turmoil of the 4th century, when the Church gathered for the first ecumenical council to respond to the crisis created by the Arian heresy.
Pope Leo XIV drew a direct line from the ancient fathers’ battle for doctrinal clarity to the modern believer’s responsibility to give a joyful, coherent witness in a fractured world: “Christians have been called to walk in harmony, guarding and transmitting the gift they have received with love and joy,” he begins, pointing to the Nicene profession of faith as “the common heritage of Christians” that remains a source of unity and hope.
In the brief letter, the Pontiff retells the drama surrounding Nicaea and walks readers through the storm of theological confusion that followed Arius’ claim that Jesus was not truly God, the decisive leadership of St. Alexander of Alexandria and Bishop Hosius of Córdoba, and the Emperor Constantine’s realization that the crisis threatened the very unity of Christianity.
The Pope highlights the blunt clarity with which the Council Fathers rejected Arius’ teaching by confessing Christ as “true God from true God… consubstantial with the Father.”
Just as in 325, the Pope argues, the Creed stands as a lifeline. The Nicene-Constantinopolitan Creed, recited every Sunday; but professing the creed is not enough. Christian witness collapses when the creed is spoken from the lips but not lived in the heart.
“What we profess with our mouths must come from the heart, so that we may bear witness with our lives,” he writes.
While the letter strongly reaffirms the full divinity of Christ and the binding authority of the Nicene Creed, Pope Leo repeatedly urges believers to defend the faith not with anger or ideological weapons, but with clarity rooted in love.
When the Church says homoousios — “consubstantial” — she is not playing with philosophy, he writes. She is protecting the truth that God Himself came among us, emptied Himself, washed our feet, suffered for us, and united Himself with every human being, Pope Leo observes.
“The Nicene Creed does not formulate a philosophical theory. It professes faith in the God who redeemed us through Jesus Christ,” the Pontiff writes, but he also highlights that “the ascent to God passes through descent and dedication to our brothers and sisters, especially the least, the poorest, the abandoned and the marginalized.”
In other words: orthodoxy and charity are inseparable. Without love, defense of the faith becomes brittle. Without truth, charity becomes sentimentality.
The letter concludes with a moving invocation of the Holy Spirit: “Come, Love of the Father and the Son… gather us into the one flock of Christ… so that we become once again what we are in Christ: one, so that the world may believe.”
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