LEO XIV

LEO XIV

APOSTOLIC LETTER

IN UNITATE FIDEI

ON THE 1700th ANNIVERSARY OF THE COUNCIL OF NICAEA

1. In the unity of faith, proclaimed since the beginning of the Church, Christians have been called to walk in harmony, guarding and transmitting the gift they have received with love and joy. This is expressed in the words of the Creed, “I believe in one Lord Jesus Christ, the Only Begotten Son of God… for our salvation he came down from heaven,” that were formulated 1700 years ago by the Council of Nicaea, the first ecumenical gathering in the history of Christianity.

As I prepare for my Apostolic Journey to Türkiye, I would like this Letter to encourage the whole Church to renew her enthusiasm for the profession of faith. For centuries, this enduring confession of faith has been the common heritage of Christians, and it deserves to be professed and understood in ever new and relevant ways. To this end, a significant document by the International Theological Commission was approved: Jesus Christ, Son of God, Saviour. 1700th Anniversary of the Ecumenical Council of Nicaea. I mention this document because it provides valuable insights for studying the importance and relevance of the Council of Nicaea, not only in its theological and ecclesial dimensions, but also in its cultural and social aspects.

2. “The beginning of the gospel of Jesus Christ, the Son of God”: with these words Saint Mark begins his Gospel, summarizing its entire message in the affirmation of Jesus Christ’s divine sonship. Similarly, the Apostle Paul knows that he is called to proclaim God’s good news concerning his Son who died and rose again for us (cf. Rom 1:9). Indeed, Jesus is God’s definitive “yes” to the promises of the prophets (cf. 2 Cor 1:19-20). In Jesus Christ, the Word, who was God before time, through whom all things were made — as the prologue of Saint John’s Gospel says — “became flesh and dwelt among us” (Jn 1:14). In him, God became our neighbor, to the extent that whatever we do to any of our brothers and sisters, we do to him (cf. Mt 25:40).

In this Holy Year, dedicated to the theme of Christ our hope, it is a providential coincidence that we are also celebrating the 1700th anniversary of the First Ecumenical Council of Nicaea, which in 325 proclaimed the profession of faith in Jesus Christ, Son of God. This is the heart of the Christian faith. Even today, during every Sunday Eucharistic celebration, we recite the Nicene-Constantinopolitan Creed, the profession of faith that unites all Christians. In these difficult times we are living, amid so many concerns and fears, threats of war and violence, natural disasters, grave injustices and imbalances, and the hunger and misery suffered by millions of our brothers and sisters, this Creed gives us hope.

3. The times of the Council of Nicaea were no less turbulent. When it began in 325, the wounds inflicted by the persecutions of Christians were still fresh. The Edict of Milan (313), issued by the emperors Constantine and Licinius, had seemed to herald the dawn of a new era of peace. However, in the wake of external threats, disputes and conflicts soon arose within the Church.

Arius, a priest from Alexandria in Egypt, taught that Jesus was not truly the Son of God. Though more than a mere creature, he was believed to be an intermediate being between the inaccessible God and humanity. Moreover, there would have been a time when the Son “did not exist.” This view was in line with the prevailing mindset of the time and therefore seemed plausible.

However, God does not abandon his Church. He always raises up courageous men and women who bear witness to the faith, as well as shepherds who guide his people and show them the way of the Gospel. Bishop Alexander of Alexandria realized that Arius’ teachings were not at all consistent with Sacred Scripture. Since Arius was not conciliatory, Alexander summoned the bishops of Egypt and Libya to a Synod, which condemned Arius’ teachings. He then sent a letter to the other bishops of the East providing a detailed report. In the West, it was Bishop Hosius of Cordoba, Spain, who took action. He had already proven himself a fervent confessor of the faith during the persecution of Emperor Maximian and enjoyed the trust of the Bishop of Rome, Pope Sylvester.

However, Arius’ followers also rallied together. This led to one of the greatest crises in the Church’s first millennium. The reason for the dispute was not a minor detail. It concerned the essence of the Christian faith, namely the answer to the decisive question that Jesus had asked his disciples at Caesarea Philippi: “Who do you say that I am?” (Mt 16:15).

4. As the controversy raged on, Emperor Constantine realized that the unity of the Church, and indeed the Empire itself, was in danger. He therefore summoned all the bishops to an ecumenical, or universal, council in Nicaea to restore unity. The Synod, known as the “Synod of the 318 Fathers,” was presided over by the emperor, and the number of bishops gathered together was unprecedented. Some of them still bore the marks of the torture they had suffered during the persecution. The vast majority of them came from the East, while it seems that only five were Westerners. Pope Sylvester entrusted the task to the theologically authoritative figure of Bishop Hosius of Cordoba and sent two Roman presbyters.

5. The Council Fathers bore witness to their fidelity to Sacred Scripture and Apostolic Tradition, as professed at baptism in accordance with Jesus’ command: “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit” ( Mt 28:19). In the West, various formulas already existed, including the one known as the Apostles’ Creed. [1] In the East, too, there were many baptismal professions similar in structure. The language used was not erudite or complicated, but rather — as was later affirmed — simple and understandable to the fishermen of the Sea of Galilee.

In light of this, the Nicene Creed begins with the following profession of faith: “I believe in one God, the Father almighty, maker… of all things visible and invisible.” [2] In this way, the Council Fathers expressed their faith in the one and only God. This point was uncontested during the Council. However, a second article was the subject of dispute. It too was based on biblical language and professed faith in one Lord, Jesus Christ, the Son of God. The debate arose from the need to address the question raised by Arius regarding how “Son of God” should be understood and how it could be reconciled with biblical monotheism. The Council therefore aimed to define the correct meaning of faith in Jesus as “the Son of God.”

The Fathers confessed that Jesus is the Son of God inasmuch as he is of the substance ( ousia) of the Father... “begotten, not made, consubstantial ( homooúsios) with the Father.” This definition was a radical rejection of Arius’ thesis. [3] In order to express the truth of the faith, the Council adopted two words — “substance” ( ousia) and “consubstantial” ( homooúsios) — which are not found in Scripture. The Council’s intention in doing so was not to replace biblical statements with Greek philosophy. On the contrary, the Council used these terms precisely to affirm biblical faith with clarity and to distinguish it from Arius’ error, which was deeply influenced by Hellenism. For this reason, the accusation of Hellenization should be directed at the false doctrine of Arius and his followers, not the Fathers of Nicaea.

The Fathers of Nicaea were firm in their resolution to remain faithful to biblical monotheism and the authenticity of the Incarnation. They wanted to reaffirm that the one true God is not inaccessibly distant from us, but on the contrary has drawn near and has come to encounter us in Jesus Christ.

6. In order to convey its message in the simple language of the Bible and the liturgy familiar to the entire People of God, the Council incorporated some expressions from the baptismal profession: “God from God, Light from Light, true God from true God.” The Council hereby adopted the biblical metaphor of light: “God is light” (1 Jn 1:3; cf. Jn 1:4-5). Just as light radiates and communicates itself without diminishing, so the Son is the reflection (apaugasma) of God’s glory and the imprint (character) of his being (hypostasis) (cf. Heb 1:3; 2 Cor 4:4). The incarnate Son, Jesus, is therefore the light of the world, and the light of life (cf. Jn 8:12). Through baptism, the eyes of our hearts are enlightened (cf. Eph 1:18), so that we too may be a light in the world (cf. Mt 5:14).

Moreover, the Creed affirms that the Son is “true God from true God.” In many places, the Bible distinguishes lifeless idols from the true and living God. The true God is the God who speaks and acts in the history of salvation: the God of Abraham, Isaac and Jacob; the God who revealed himself to Moses in the burning bush (cf. Ex 3:14); the God who sees the misery of the people, hears their cry, and guides and accompanies them through the desert in the pillar of fire (cf. Ex 13:21); the God who speaks to them with a voice of thunder (cf. Deut 5:26) and has compassion on them (cf. Hos 11:8-9). Christians are therefore called to turn away from lifeless idols to the living and true God (cf. Acts 12:25; 1 Thess 1:9). To this end, Simon Peter proclaims at Caesarea Philippi: “You are the Christ, the Son of the living God” (Mt 16:16).

7. The Nicene Creed does not formulate a philosophical theory. It professes faith in the God who redeemed us through Jesus Christ. It is about the living God who wants us to have life and to have it in abundance (cf. Jn 10:10). For this reason, the Creed then continues with the words of the baptismal profession: the Son of God who “for us men and for our salvation… came down from heaven, and… became man… suffered death… and rose again on the third day… ascended into heaven… and will come again… to judge the living and the dead.” It is thus clear that the Council’s statements regarding faith in Christ are rooted in the history of salvation between God and his creatures.

Saint Athanasius, who had participated in the Council as deacon to Bishop Alexander and later succeeded him as Bishop of Alexandria in Egypt, repeatedly and effectively emphasized the soteriological dimension of the Nicene Creed. He wrote that the Son, who came down from heaven, “made us children of the Father and, deified mankind by becoming himself man. Therefore, he was not man, and then became God; but he was God, and then became man, and that to deify us.” [4] This is only possible if the Son is truly God: no mortal being can, in fact, defeat death and save us; only God can do so. He has freed us through his Son made man, so that we might be free (cf. Gal 5:1).

It is worth emphasizing the verb descendit, in the Nicene Creed: “he came down.” Saint Paul describes this movement in strong terms: “[Christ] emptied himself, taking the form of a slave, being born in human likeness” (Phil 2:7). The prologue to the Gospel of Saint John likewise states that “the Word became flesh and dwelt among us” (Jn 1:14). The Letter to the Hebrews also teaches that “we do not have a high priest who is unable to sympathize with our weaknesses, but we have one who in every respect has been tested as we are, yet without sin” (Heb 4:15). On the eve of his death, he humbled himself like a slave to wash the feet of his disciples (cf. Jn 13:1-17). Only when he was able to put his fingers into the wound of the risen Lord’s side did the Apostle Thomas confess: “My Lord and my God!” (Jn 20:28).

It is precisely by virtue of his Incarnation that we now encounter the Lord in our brothers and sisters in need: “As you did it to one of the least of these brothers and sisters of mine, you did it to me” (Mt 25:40). The Nicene Creed does not depict a distant, inaccessible and immovable God who rests in himself, but a God who is close to us and accompanies us on our journey in the world, even in the darkest places on earth. His immensity is revealed when he makes himself small, laying aside his infinite majesty to become our neighbor in the little ones and in the poor. This revolutionizes pagan and philosophical conceptions of God.

Another phrase from the Nicene Creed is also particularly revealing for us today. The biblical statement “became flesh” is clarified by adding the word “man” after “incarnate.” Nicaea thus distances itself from the false doctrine that the Logos took on only a body as an outer covering and not the human soul, which is endowed with intellect and free will. Instead, it seeks to affirm what the Council of Chalcedon (451) would later explicitly declare: in Christ, God assumed and redeemed the whole human being, body and soul. Saint Athanasius explains that the Son of God became man so that man might be deified. [5] This enlightening understanding of divine revelation was prepared by Saint Irenaeus of Lyon and Origen, and then further developed with great richness in Eastern spirituality.

Divinization in no way implies the self-deification of man. On the contrary, divinization protects us from the primordial temptation to want to be like God (cf. Gen 3:5). What Christ is by nature, we become by grace. Through the work of redemption, God not only restored our human dignity as his image, but the One who created us in a wondrous way, has now made us partakers in his divine nature in an even more wondrous way (cf. 2 Pet 1:4).

Divinization, then, is true humanization (becoming fully human). This is why human existence points beyond itself, seeks beyond itself, desires beyond itself, and is restless until it rests in God. [6] “ Deus enim solus satiat, God alone satisfies man!” [7] Only God, in his infinity, can satisfy the infinite desire of the human heart, and for this reason the Son of God chose to become our brother and redeemer.

8. As we have already said, Nicaea clearly rejected the teachings of Arius. However, Arius and his followers did not give up. The Emperor Constantine himself and his successors increasingly sided with the Arians. The term homooúsios became a bone of contention between the Nicene and anti-Nicene factions, thus triggering other serious conflicts. Saint Basil of Caesarea eloquently described the ensuing confusion by likening it to a nighttime naval battle in a violent storm. [8] Saint Hilary, on the other hand, testified to the orthodoxy of the laity in contrast to the Arianism of many bishops, acknowledging that “the ears of the people are holier than the hearts of the priests.” [9]

Saint Athanasius became the firm foundation of the Nicene Creed through his unyielding and steadfast faith. Although he was deposed and expelled from the Episcopal See of Alexandria five times, he returned each time as bishop. Even while in exile, he continued to guide the People of God through his writings and letters. Like Moses, Athanasius was unable to enter the promised land of ecclesial peace. This grace was reserved for a new generation, known in some places as the “Nicene youth.” In the East, this generation included the three Cappadocian Fathers: Saint Basil of Caesarea (c. 330-379), who was given the title “the Great;” his brother Saint Gregory of Nyssa (335-394); and Basil’s greatest friend, Saint Gregory Nazianzen (329/30-390). In the West, significant figures include Saint Hilary of Poitiers (c. 315-367), his student Saint Martin of Tours (c. 316-397) and, above all, Saint Ambrose of Milan (333-397) and Saint Augustine of Hippo (354-430).

The particular merit of the three Cappadocians was bringing to completion the formulation of the Nicene Creed by showing that, in God, Unity and Trinity are in no way contradictory. This development led to the formulation of the article of faith concerning the Holy Spirit at the First Council of Constantinople in 381. Consequently, the Creed took the name “Nicene-Constantinopolitan Creed,” and now states: “I believe in the Holy Spirit, the Lord, the giver of life, who proceeds from the Father, who with the Father and the Son is adored and glorified, who has spoken through the prophets.” [10]

At the Council of Chalcedon in 451, the Council of Constantinople was recognized as ecumenical, and the Nicene-Constantinopolitan Creed was declared to be universally binding. [11] It therefore constituted a bond of unity between the East and the West. In the 16 century, it was also upheld by the ecclesial communities that arose from the Reformation. The Nicene-Constantinopolitan Creed is thus the common profession of all Christian traditions.

9. The path that began with Sacred Scripture and led to the profession of faith in Nicaea, subsequently accepted in Constantinople and Chalcedon, and again in the 16th and 21st centuries, has been a long and consistent one. All of us, as disciples of Jesus Christ, are baptized “in the name of the Father, and of the Son and of the Holy Spirit.” We make the sign of the cross on ourselves and we are blessed. We conclude each prayer of the psalms in the Liturgy of the Hours with “Glory be to the Father, and to the Son, and to the Holy Spirit.” Both the liturgy and the Christian life are thus firmly anchored in the Nicene-Constantinopolitan Creed: what we profess with our mouths must come from the heart so that we may bear witness to it with our lives. We must therefore ask ourselves: What about our interior reception of the Creed today? Do we experience that it also affects our current situation? Do we understand and live out what we say every Sunday? What do these words mean for our lives?

10. The Nicene Creed begins by professing faith in God, the Almighty, the maker of heaven and earth. For many people today, however, God and the question of God have almost no meaning in their lives. The Second Vatican Council pointed out that Christians are at least partly responsible for this situation, because they do not bear witness to the true faith; they hide the true face of God with lifestyles and actions that diverge from the Gospel. [12] Wars have been fought, and people have been killed, persecuted and discriminated against in the name of God. Instead of proclaiming a merciful God, a vengeful God has been presented who instils terror and punishes.

In this sense, the Nicene Creed invites us to examine our conscience. What does God mean to me and how do I bear witness to my faith in him? Is the one and only God truly the Lord of my life, or do I have idols that I place before God and his commandments? Is God for me the living God, close to me in every situation, the Father to whom I turn with filial trust? Is he the Creator to whom I owe everything I am and have, whose mark I can find in every creature? Am I willing to share the goods of the earth, which belong to everyone, in a just and equitable manner? How do I treat creation, the work of his hands? Do I exploit and destroy it, or do I use it with reverence and gratitude, caring for and cultivating it as the common home of humanity? [13]

11. The profession of faith in Jesus Christ, our Lord and God is the center of the Nicene-Constantinopolitan Creed. This is the heart of our Christian life. For this reason, we commit to follow Jesus as our master, companion, brother and friend. But the Nicene Creed asks for more: it reminds us not to forget that Jesus Christ is the Lord (Kyrios), the Son of the living God who “for our salvation came down from heaven” and died “for our sake” on the cross, opening the way to new life for us through his resurrection and ascension.

Naturally, following Jesus Christ is not a wide and comfortable path. However, this often demanding or even painful path always leads to life and salvation (cf. Mt 7:13-14). The book of the Acts of the Apostles recounts the new way (cf. Acts 19:9, 23; 22:4, 14-15, 22) that is Jesus Christ (cf. Jn 14:6). Following the Lord necessarily entails following the way of the cross, which, through repentance, leads us to sanctification and divinization. [14]

If God loves us with all his being, then we too must love one another. We cannot love God whom we do not see without loving our brother and sister whom we do see (cf. 1 Jn 4:20). Love for God without love for neighbor is hypocrisy; radical love for our neighbor, especially love for our enemies, without love for God, requires a “heroism” that would overwhelm and oppress us. In following Jesus, the ascent to God passes through descent and dedication to our brothers and sisters, especially the least, the poorest, the abandoned and the marginalized. What we have done to the least of these, we have done to Christ (cf. Mt 25:31-46). In the face of disasters, wars and misery, we bear witness to God’s mercy to those who doubt him only when they experience his mercy through us. [15]

12. Finally, the Council of Nicaea is relevant today because of its great ecumenical value. Indeed, the achievement of unity among all Christians was one of the main objectives of the last Council, the Second Vatican Council. [16] Exactly thirty years ago, Saint John Paul II further promoted this conciliar message in his Encyclical Ut Unum Sint (25 May 1995). In this way, together with the great anniversary of the First Council of Nicaea, we also celebrate the anniversary of the first ecumenical Encyclical. It can be considered a manifesto that brought up to date the same ecumenical foundations laid down by the Council of Nicaea.

Thanks to God, the ecumenical movement has achieved much in the last sixty years. It is true that full visible unity with the Orthodox and Eastern Orthodox Churches and with the ecclesial communities born of the Reformation has not yet been reached. Nevertheless, ecumenical dialogue, founded on one baptism and the Nicene-Constantinopolitan Creed, has led us to recognize the members of other Churches and ecclesial communities as our brothers and sisters in Jesus Christ, and to rediscover the one universal community of Christ’s disciples throughout the world. We share the same faith in the one and only God, the Father of all people; we confess together the one Lord and true Son of God, Jesus Christ, and the one Holy Spirit, who inspires us and impels us towards full unity and the common witness to the Gospel. Truly, what unites us is much greater than what divides us! [17] In a world that is divided and torn apart by many conflicts, the one universal Christian community can be a sign of peace and an instrument of reconciliation, playing a decisive role in the global commitment to peace. Saint John Paul II reminded us, in particular, of the witness of the many Christian martyrs from all Churches and ecclesial communities: their memory unites us and spurs us on to be witnesses and peacemakers in the world.

In order to carry out this ministry credibly, we must walk together to reach unity and reconciliation among all Christians. The Nicene Creed can be the basis and reference point for this journey. It offers us a model of true unity in legitimate diversity. Unity in the Trinity, Trinity in Unity, because unity without multiplicity is tyranny, multiplicity without unity is fragmentation. The Trinitarian dynamic is not a dualistic and exclusive “either/or,” but rather a decisive bond, “both/and.” The Holy Spirit is the bond of unity whom we worship together with the Father and the Son. We must therefore leave behind theological controversies that have lost their raison d’être in order to develop a common understanding and even more, a common prayer to the Holy Spirit, so that he may gather us all together in one faith and one love.

This does not imply an ecumenism that attempts to return to the state prior to the divisions, nor is it a mutual recognition of the current status quo of the diversity of Churches and ecclesial communities. Rather, it is an ecumenism that looks to the future, that seeks reconciliation through dialogue as we share our gifts and spiritual heritage. The restoration of unity among Christians does not make us poorer; on the contrary, it enriches us. As at Nicaea, this goal will only be possible through a patient, long and sometimes difficult journey of mutual listening and acceptance. It is a theological challenge and, even more so, a spiritual challenge, which requires repentance and conversion on the part of all. For this reason, we need the spiritual ecumenism of prayer, praise and adoration, as expressed by the Creed of Nicaea and Constantinople.

Let us therefore invoke the Holy Spirit to accompany and guide us in this work.

Holy Spirit of God, you guide believers along the path of history.

We thank you for inspiring the Symbols of Faith and for stirring in our hearts the joy of professing our salvation in Jesus Christ, the Son of God, consubstantial with the Father. Without him, we can do nothing.

Eternal Spirit of God, rejuvenate the faith of the Church from age to age. Help us to deepen it and to return always to the essentials in order to proclaim it.

So that our witness in the world may not be futile, come, Holy Spirit, with your fire of grace, to revive our faith, to enkindle us with hope, to inflame us with charity.

Come, divine Comforter, source of harmony, unite the hearts and minds of believers. Come and grant us to taste the beauty of communion.

Come, Love of the Father and the Son, gather us into the one flock of Christ.

Show us the ways to follow, so that with your wisdom, we become once again what we are in Christ: one, so that the world may believe. Amen.

From the Vatican, 23 November 2025, Solemnity of Our Lord Jesus Christ, King of the Universe

LEO PP. XIV

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[1] L.H. Westra, The Apostles’ Creed. Origin, History and Some Early Commentaries, Turnhout 2002 ( Instrumenta patristica et mediaevalia, 43).

[2] First Council of Nicaea, Expositio fidei: CC COGD 1, Turnhout 2006, 19 6-8.

[3] Saint Athanasius of Alexandria, Contra Arianos, I, 9, 2 (ed. Metzler, Athanasius Werke, I/1,2, Berlin - New York 1998, 117-118) From the statements of Saint Athanasius in Contra Arianos I, 9, it is clear that homooúsios does not mean “of similar substance,” but “of the same substance” as the Father; it is therefore not a question of similarity of substance, but of identity of substance between Father and Son. The Latin translation of homooúsios therefore rightly speaks of unius substantiae cum Patre.

[4] Saint Athanasius of Alexandria, Contra Arianos, I, 38, 7 - 39, 1: ed. Metzler, Athanasius Werke, I/1,2, 148-149.

[5] Saint Athanasius of Alexandria, Cf. De incarnatione Verbi, 54, 3: SCh 199, Paris 2000, 458; id., Contra arianos, I, 39; 42; 45; II, 59ss.: ed. Metzler, Athanasius Werke, I/1,2, 149; 152, 154-155 e 235ss.

[6] Cf. Saint Augustine, Confessions, I, 1: CCSL 27, Turnhout 1981, 1.

[7] Saint Thomas Aquinas, In Symbolum Apostolorum, art. 12: ed. Spiazzi, Thomae Aquinatis, Opuscula theologica, II, Taurini - Romae 1954, 217.

[8] Saint Basil, De Spiritu Sancto, 30.

[9] Saint Hilary, Contra Arianos, vel Auxentium, 6. Mindful of the voices of the Fathers, the learned theologian, later Cardinal and now Saint and Doctor of the Church, John Henry Newman (1801-1890), investigated this dispute and came to the conclusion that the Nicene Creed has been preserved above all by the sensus fidei of the people of God. See On Consulting the Faithful in Matters of Doctrine (1859).

[10] First Council of Constantinople, Expositio fidei: CC, Conc. Oec. Gen. Decr. 1, 57 20-24. The statement “and proceeds from the Father and the Son ( Filioque)” is not found in the text of Constantinople; it was inserted into the Latin Creed by Pope Benedict VIII in 1014 and is a subject of Orthodox-Catholic dialogue.

[11] Council of Chalcedon, Definitio fidei: CC, Conc. Oec. Gen. Decr. 1, 137 393-138 411.

[12] Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et spes, 19: AAS 58 (1966), 1039.

[13] Cf. Francis, Encyclical Letter Laudato Si’ (24 May 2015), 67; 78; 124: AAS 107 (2015), 873-874; 878; 897.

[14] Cf. Francis, Apostolic Exhortation Gaudete et Exsultate (19 March 2018), 92: AAS 110 (2018), 1136.

[15] Cf. Francis, Encyclical Letter Fratelli Tutti (3 October 2020), 67; 254: AAS 112 (2020), 992-993; 1059.

[16] Cf. Second Vatican Ecumenical Council, Decree on Ecumenism Unitatis Redintegratio, 1: AAS 57 (1965), 90-91.

[17] Cf. Saint John Paul II, Encyclical Letter Ut Unum Sint (25 May 1995), 20: AAS 87 (1995), 933.

The Holy See