In the One, We Are One: Christian Unity in Augustine’s City of God
In the year 410, the Eternal City, Rome, was sacked by barbarians, giving the lie to the idea that the city was in fact eternal. St. Jerome rather dramatically sums up what many people felt:
When the bright light of all the world was put out, or, rather, when the Roman Empire was decapitated . . . the whole world perished in one city. Who would believe that Rome, built up by the conquest of the whole world, had collapsed, that the mother of all nations became their tomb? (Commentary on Ezekiel, Preface).
It was like the psychological shock of 9/11. This just does not happen here. Many educated pagans and aristocratic Christians blamed Christianity. Christians were upstarts who abandoned the traditional gods who had kept Rome secure for centuries. The gods were punishing the Romans for embracing Christianity.
It was these complaints that led Augustine to write the City of God. In this “great and arduous work” of 1000 pages, Augustine sets out to refute these accusations, which he does mostly in the first fifty pages. But once he got started, Augustine saw that his real project was deeper: he wanted to definitively refute the whole pagan project and its idolatry while showing how Christianity offered a true path to salvation through worship of the one true God.
One part of this polemic was to reorient what people understood a “city” to be. On a basic level, the Romans had a similar idea as we do: a city is a place, usually with a high concentration of people and power. This is why the invasion of Rome was so devastating: understood not just as a city but as the eternal city, the violation of its boundaries was experienced like a personal violation, a rape, which defiled “the mother of all nations.” But Augustine shifts the terms of the debate. Identifying the place of Rome as an eternal city is simply idolatry. There is only one eternal city: God’s. Physical boundaries do not make a city; rather—as Augustine defines it—a city “as a multitude of people in concord” (City of God, 1.15).
If a city is a “multitude of people in concord,” then we can begin to understand what Augustine means by using the term “city of God.” We also have a bit of the background for Pope Leo XIV’s Papal motto, in illo uno unum, “in the one we are one,” which he borrowed from Augustine’s Expositions on the Psalms. In the context of the original phrase, Augustine is speaking about one of the most beautiful aspects of his theology: the totus Christus, or the whole Christ. In context, the papal motto reads, “Although we Christians are many, in the one Christ we are one.” The “one Christ” or the “whole Christ” is Christ both head and body. Christ the head is risen and ascended into heaven; the body is all of us who inseparably make up one organism with Christ.
Augustine draws on the famous story of Paul’s conversion (Acts 9). Christ appears to Saul and asks, “Saul, Saul, why are you persecuting me?” When Saul asks who is speaking, Christ answers, “I am Jesus whom you are persecuting.” But Saul is persecuting Christians. Christians are Christ. Christ’s body is him. There is no separation. Just as if I were to come over and kick you in the shins, you would say, “Don’t kick me.” You are your body. So, too, Christ is one with his body.
But to understand this beautiful vision of oneness, we should dive a little deeper into the City of God. What does this great work teach us about what Christian unity means? How does Augustine—and therefore Pope Leo—understand what it means to be one? What does this look like for us? And what might this mean for some of Pope Leo’s concerns about AI and transhumanism?
Two Loves, Two Cities
There are only two kinds of people in the world: those who divide people into two kinds of people and those who do not. Augustine is of the first kind. Of all the diversity of cities, nations, customs, races, languages, all people belong to one of two cities: the city of God or the earthly city. A city, remember, is a “multitude of people who live in concord,” that is, the city one belongs to is determined by what loves they share. In his discussion of the creation and fall of the angels, Augustine says:
We see then that the two cities were created by two kinds of love: the earthly city was created by self-love reaching the point of contempt for God, the Heavenly City by the love of God carried as far as contempt of self. In fact, the earthly city glories in itself, the Heavenly City glories in the Lord. The former looks for glory from men, the latter finds its highest glory in God, the witness of a good conscience (City of God, 14.28).
When God said, Let there be light, he created the angels. But some turned away from him and became dark, so God separated the light from the darkness. God created all angels good and to be illumined by him, but some did not want to receive their light from God and wanted to be the source of their own light. These fell and became the devil and his demons. Some turned to God in love, while others turned away in self-love. Thus, from the very beginning, there were two cities created by two loves.
What happened in the case of the angels also happened for human beings. Original man and woman were created good, but seduced by the devil they wanted to be gods without God. They reached for divinity on their own terms. Thus, they fell. In Cain and Abel we see the historical roots of the two cities on earth: the fratricide and the martyr; the one who gave himself the first fruits and the one who gave God his first fruits. The one who killed to be on top and the one who like a lamb was led to the slaughter.
So, all angels and humans are divided into two cities: the city of God, also called the heavenly city, and the earthly city. The heavenly city is first and created by God. The earthly city is founded by the devil. Following this, there are only two ways to live for Augustine: according to the spirit or according to the flesh; according to God or according to the self. If we live according to God, then we become like God. If we live according to the self, we become like the devil.
The two different loves lead to two different kinds of unity. In imitation of the devil, the love of self leads to what Augustine calls the libido dominandi, the lust for dominating or, better, the dominating lust. The earthly city is marked by the libido dominandi, but there is an irony: lust for dominating dominates the dominator so they become subjects as they subject others. This is part of God’s in-built punishment. For Augustine, the libido dominandi is found in its most undiluted form in Roman people. They conquered the whole world and forcibly made them Roman, as the quote from Jerome shows well, “Who would believe that Rome, built up by the conquest of the whole world, had collapsed, that the mother of all nations became their tomb?” They unify the world by conquest, by enslaving others and bringing them under their military, legal, and cultural power.
But the love of God brings about a different kind of unity. This is a unity found in the city of God, angels and humans bound together in their shared love.
God’s City
The very first passage of the City of God tells us important things about the heavenly city. Augustine writes,
Here, my dear Marcellinus, is the fulfilment of my promise, a book in which I have taken upon myself the task of defending the glorious City of God against those who prefer their own gods to the Founder of that City. I treat of it both as it exists in this world of time, a stranger among the ungodly, living by faith, and as it stands in the security of its everlasting seat (City of God, 1.1).
So, you see the two cities here, but you also see something else: the one city of God has two dimensions. There is one city of God in heaven and on earth; in eternity and in time; in security and in faith; at home and as a stranger. There is one city of God with two dimensions: in heaven there are angels and saints who enjoy God face to face in eternal security and happiness; but on earth there are also believers who are joined to that heavenly city even as they experience the trials of this life.
The city of God on earth is the Church. But it is not simply identical to the Church. The Church is the Body of Christ, but it is a mixed body.
Such is the reply which the redeemed household of servants of the Lord Christ—the pilgrim City of Christ the King—may return to its enemies. She must bear in mind that among these very enemies are hidden her future citizens; and when confronted with them she must not think it a fruitless task to bear with their hostility until she finds them confessing the faith. In the same way, while the City of God is on pilgrimage in this world, she has in her midst some who are united with her in participation in the sacraments, but who will not join with her in the eternal destiny of the saints. . . . But, such as they are, we have less right to despair of the reformation of some of them, when some predestined friends, as yet unknown even to themselves, are concealed among our most open enemies. In truth, those two cities are interwoven and intermixed in this era, and await separation at the last judgement (City of God, 1.35).
The Church is a mix of good and bad, of wheat and weeds (see Matthew 13). Jesus himself tells us that we cannot uproot the wicked (or not so good) right now lest we uproot those who might convert later. And neither should we be too confident about our own status as wheat. We, too, might change, for the worse. People move back and forth between the city of God (at least, from our limited perspective) based on their fundamental allegiance and primary love (so, from God’s perspective we may have always been in one city or the other). We should hope for the conversion of our enemies (both inside and outside the Church) and pray for our own perseverance to the end. If such a radical enemy of the Church like Saul could become the greatest apostle, then we should not despair of anyone.
This makes the oneness of the Church complicated. We can be one in the sacraments but not one in destiny. We can be visibly one but not invisibly one. We can never be sure because we cannot know the heart and even our own hearts we cannot trust will be the same tomorrow. God will sort it all out in the end.
But we can say that the true Church, whoever constitutes it, is truly one with the heavenly Church. They form one city of God for they are “a multitude of people in concord.” Con-cord, by the way, means “hearts together.” The true citizens of the city of God all love God above all things and this love unites them.
So, the two dimensions of the city of God have a unity, but there is also a unity through time. To show this, Augustine uses the striking image of the birth of Jacob and Esau (see On the Catechizing of the Uninstructed, 1.6). Recall the story of Jacob and Esau’s birth (Genesis 25): they are twins who wrestled in the womb of their mother Rebecca. Esau emerges first with Jacob grasping his heal. Christ is the Head of the Church, but he does not appear first. Rather, like Jacob being born, the hand (Israel) appears first, tripping up Esau (the nations). Then emerges the Head (Christ) and finally the body (the Church). All the faithful, old and new, are one body. The Jews emerge first in time announcing the coming of Christ, but they are organically connected to the body, which emerges after the Head and is subordinate to it.
Ecclesial Unity: Defined by the Mercy of Christ
Augustine’s vision of unity is profoundly Christological and his theology of Christ is framed by the idea of mercy.[1] For Augustine, the Incarnation is the ultimate act of mercy. This culminates in the crucifixion where Christ offers the perfect sacrifice to the Father and in doing so offers the perfect act of worship. From this perfect sacrifice, this supreme act of mercy, the Church is born: the blood and water that pour from the side of Christ symbolize the Eucharist and baptism, the two sacraments that constitute us as Christ’s body. Augustine here is drawing on a common patristic reading of the crucifixion scene in John’s Gospel. Christ is presented as the New Adam who, like the old Adam, “falls asleep” and from whose “side” God creates the Woman, the bride of Adam. For Augustine, this moment defines the whole reality of the Church. We are saved by Christ’s mercy. We are made new by Christ’s mercy. Our whole Christian existence is defined by Christ’s mercy.
This means that in order to achieve our final end—to be in full unity with God and each other—we must practice mercy. It is worth quoting Augustine at length here:
Thus the true sacrifice is offered in every act which is designed to unite us to God in a holy fellowship, every act, that is, which is directed to that final Good which makes possible our true felicity. . . . So then, the true sacrifices are acts of compassion [or mercy], whether towards ourselves or towards our neighbors, when they are directed towards God; and acts of compassion are intended to free us from misery and thus to bring us to happiness (City of God, 10.6).
Augustine pithily defines mercy (misericordia) as “a kind of compassion (compassio) in our hearts for the misery (miseriae) of another that compels us to come to his aid if we can” (City of God, 9.5). To relieve someone’s misery we must aid them in ways that truly help them. Certainly, this means the corporal works of mercy (see Matthew 25), but even these are not ends in themselves. “Even the mercy which we extend to human beings,” Augustine says, “is not a sacrifice if it is not done for God’s sake” (City of God, 10.6). Works of mercy must be undertaken with our—and our neighbor’s—ultimate end in mind. We must aid people in ways that help us and them achieve union with God, the only thing that can make any of us happy. “The true sacrifice,” Augustine says, “is every act done in order that we might cling to God in holy fellowship” (City of God, 10.6).
We cannot say that we love God if we do not love our neighbor. Yet, we also cannot say we love our neighbor if we do not love him in a way that will bring him and us to true happiness. We are commanded to love our neighbor as ourselves. To love ourselves means to will our own good, to seek true happiness. It means to have mercy on ourselves by doing the things that will relieve our misery, that is, the things that will aid us in clinging to God.
We have mercy on ourselves by making our lives into a sacrifice. “Our body is also a sacrifice,” Augustine says, “when we discipline it with temperance” so that we can present “our bodies as a living sacrifice, holy and pleasing to God” (City of God, 10.6, quoting Romans 12:1). We offer our soul as a sacrifice “when it directs itself to God so that, aflame with the fire of love for him, it loses the form of worldly desire and, now subject to him, is reformed to him as to an unchanging form, thus pleasing him by receiving its beauty from his beauty” (City of God, 10.6). We show true mercy, and therefore true love, for our neighbor when we relieve their misery in the same way: by helping them to make their bodies and souls a sacrifice to God.
For Augustine, this is not just an individualistic vision of salvation. Augustine goes on in the same passage to describe how we enter into Christ’s act of mercy in the Eucharist:
This being so, it immediately follows that the whole redeemed community, that is to say, the congregation and fellowship of the saints, is offered to God as a universal sacrifice, through the great Priest who offered himself in his suffering for us—so that we might be the body of so great a Head . . . so we are many, but we make up one body in Christ; and individually we are members of one another, possessing gifts differing according to the grace which has been given us. This is the sacrifice of Christians, who are “many, making up one body in Christ.” This is the sacrifice which the Church continually celebrates in the sacrament of the altar, a sacrament well-known to the faithful where it is shown to the Church that she herself is offered in the offering which she presents to God (City of God, 10.6).
In Scripture, the phrase “body of Christ” has three meanings. It refers to Christ’s physical incarnate body (born, crucified, risen); it refers to the Eucharist; and it refers to the Church. Although some people see these are three distinct realities, Augustine tends to treat them as one mystical reality.
For Augustine, baptism establishes a radical identity with Christ. In a sermon, Augustine says to his congregation, “Let us rejoice and give thanks that we have become not only Christians, but Christ himself.” In baptism, we become members of the totus Christus; we become an organic part of the whole Christ. In the Eucharist, we truly receive the body of Christ and this profoundly augments our union with the whole body.
Augustine understands each sacramental mystery to illumine the other and often suggests that there is not only a symbolic relationship but also a kind of mystical identity between the baptized congregation and the Eucharist. Augustine compares the process of making the Eucharist to the process catechumens undergo as they prepare for incorporation into the Church. The congregation is like scattered grain which is gathered together, ground in exorcism, mixed with the water of baptism, baked with the fire of the Holy Spirit in confirmation, and transformed into the eucharistic bread.
But this symbolic element is in no way separated from the reality which it signifies: in baptism, we are truly incorporated into the Body of Christ; in the Eucharist, the bread truly becomes the Body of Christ. Augustine tells his congregation, “So if you are the Body of Christ and its members, it is your mystery that has been placed on the Lord’s table; you receive your own mystery. . . . Be what you see, and receive what you are.” In the celebration of the Eucharist, the congregation offers itself up on the altar along with the bread and wine: the Body of Christ (the baptized congregation) offers itself along with and precisely as the perfect sacrifice of the Body of Christ (the Eucharist).
In the prayers leading up to the Eucharist, the priest says, “Lift up your heart,” sursum cor, and the people respond “We hold it up toward the Lord.” Augustine says, “When our heart is up to the Lord, it is his altar.” As the congregation lifts up its heart to God in the Eucharistic liturgy, it becomes an altar on which they offer themselves back to God as the Body of Christ. “Lift up the heart! This is the whole life of true Christians: Lift up the heart.” In this motion, Christian hearts become one heart in which the congregation offers the one sacrifice of themselves, mystically identical to the sacrifice of bread and wine on the altar. By partaking of the Eucharist, we become what we receive and are able to offer ourselves back to God in union with Christ’s sacrifice.
In baptism, we become one with the one Christ. And in the Eucharist this union is advanced as we are more fully conformed to Christ. Because Christ is truly present in the Eucharist, we do not transform him into us, but we are transformed into him.
Unity through Miracles
The final book of the City of God deals with the Resurrection, the end of all things. Interestingly, the first third of this last book treats of miracles, mostly miracles contemporary to Augustine which took place at the shrines of the martyrs. It will be worth reflecting on both miracles and resurrection, as both tell us something important about how Augustine understands Christian unity.
Augustine relates dozens of miracle stories that he either witnessed himself or heard from trustworthy people. Miracles show that God is still active and still active in his followers. Miracles did not stop with apostles. The miracles Augustine relates occur through relics, the bones of the martyrs. Nothing is more dead than a bone, yet nothing, at least in Augustine’s time, is more powerful and life-giving than God working through these bones.
The word martyr means witness. A witness is someone who testifies to the truth of what he or she has seen. Martyrs bear witness to truth they have seen: the truth of Christ, the truth of his resurrection. They are willing to die for this truth, to testify not just with words, but with blood. They testify to the truth that dying for Christ is not an end, but the path of true life. Martyrs attest to faith in resurrection.
In the Gospels, Jesus’ miracles testify to the Kingdom of God: take up your mat and walk, your faith has healed you. Faith heals soul; cure heals body. Both faith and the cure are miracles that Jesus calls forth to heal the whole person. They are a sign that the Kingdom of God is breaking in, that the reign of sin and death is being reversed.
The martyrs continue this work as Christ’s body on earth. But more than this, their relics become realms of truth where the two dimensions of the city of God meet. The martyrs from the security of heaven still love their neighbors on earth and so reach out to those of us on pilgrimage. The miracles they perform through their life-giving dead bones heal our ailments and encourage our faith so we can meet them one day in heaven. They show us the kingdom and how there are so-called “thin places” on earth where we experience love with those on the other side. The love the martyrs show in healing us and the gratitude we show for what they have done is another way of seeing what Christian unity means for Augustine.
Unity in the Eternal Sabbath and Resurrection
We now have a little background about what Christian unity looks like in the Church’s sacraments and with the angels and saints. But what will Christian unity look like in heaven when we are all resurrected? Augustine’s primary image for thinking about this is the eternal Sabbath rest.
For Augustine, rest is not the cessation of activity, but the coming to be what we were created to be in the place we were meant to be. A fish out of water is restless until it returns to the sea where it belongs. We are restless until we rest in God, until God makes us to be what he created us to be and we abide in him and he in us forever. Sabbath rest is also Augustine’s primary image of deification. It names our destiny of full union with God where he will be “all in all.” Let me unpack this a bit.
At the very climax of the City of God, Augustine says that not only will we enjoy a Sabbath rest in God, but that we will become the Sabbath:
We ourselves shall become that seventh day, when we have been replenished and restored by his blessing and sanctification. There we shall have leisure to be still, and we shall see that he is God, whereas we wished to be that ourselves when we fell away from him, after listening to the Seducer saying, “You will be like gods.” Then we abandoned the true God, by whose creative help we should have become gods, but by participating in him, not by deserting him. . . . But now restored by him and perfected by his greater grace we shall be still and at leisure for eternity, seeing that he is God, and being filled by him when he will be all in all (City of God, 22.30).
Here we have Augustine’s portrait of our full deification in heaven. The word deification means “to make into a god.” This is what Augustine is referring to in the quote: God wants to make us into gods, that is, into immortal and incorruptible beings permeated with his life. But we cannot do this on our own! We do this by participating in God, not by abandoning him.
As a working definition of deification, we can say that deification is the process by which the Holy Spirit unites us to the Father by conforming us to Christ. On Augustine’s understanding, deification begins in this life with the first touch of God’s grace—either in baptism or in the first stirrings of faith which lead to baptism—but is not completed until after the resurrection in the next life. Through the power of the Holy Spirit, especially in the sacraments, we are united to God and, while never ceasing to be human, our union with God transforms us into the one to whom we are united. By cooperating with God’s grace, we advance in likeness to Christ—as St. Paul says, “being changed into his likeness from one degree of glory to another” (2 Cor. 3:18)—until we see God “face to face” (1 Cor. 13:12). Then, as St. John says, “We shall be like him, for we shall see him as he is” (1 John 3:2). This likeness to God is not external or merely metaphorical: rather, we are like God because God shares himself with us and we participate in him. The early Church Fathers used the image of an iron in the fire to explain this participation in God: the iron remains iron, but it becomes so permeated with fire that it takes on the qualities of fire. So, too, when we are immersed in God, we become permeated with God and take on his qualities.
For Augustine, deification means allowing the Holy Spirit to reform us so that we are conformed to God more completely and therefore reflect him more beautifully. It means that the presence of God permeates us so thoroughly that all our actions are God’s actions in us. This transformation in God inevitably pours out into the love of neighbor, whom we must love as ourselves. This process starts in baptism when we become Christ and continues throughout our lives as we progress in God-likeness through works of mercy and participating in the Eucharist.
This process of deification will be complete when our souls are perfected in heaven and our bodies are perfected in the resurrection. Augustine says that God permeating our souls is the Sabbath Day and God permeating our bodies is the eighth day. What does this mysterious day mean?
Christ died on Good Friday, rested in the tomb on the Sabbath, and rose again on the first day of the week—what we now call “the Lord’s day.” Following an already established tradition, Augustine understood that this sequence was rich in significance. As a faithful Jew, Christ observed the Sabbath on Holy Saturday and rose “on the third day,” a biblical shorthand for theophany, the fullness of time when God appears (see Ex 19:10–16). This third day is also the first day of the week, a sign that Christ is inaugurating a new creation. Augustine understood the Lord’s day to be the “eighth day,” an allusion to the fulfillment in Christ’s resurrection of the Genesis creation story. God creates all things in six days, and each day has an evening and morning, signifying that all things are bound to time. But God rests on the seventh day, the Sabbath, which is never said to have an evening. The Sabbath day opens into an eternal eighth day that will never end. Christ rose on this eighth day, signifying the fulfillment of all things—that is, signifying the taking up of all things into divine life. For those who die in Christ, their souls experience the rest of the Sabbath, union and fullness with God. But they await the fulfillment of this in their bodies when they rise in Christ. Then all things will be fulfilled in them: they will experience God in their souls and bodies as fully divinized humans in an eighth day that will never end.
Conclusion
We have ranged far and wide over Augustine’s City of God to try to understand what Christian unity might mean for Pope Leo XIV. If Pope Leo picks up any of the echoes of Augustine, then he will offer the Church a profound vision of what it means to be one in Christ. This may serve us well as a bulwark against the coming storm. We know that Leo also is rightly concerned with the coming AI revolution which will affect culture, society, and the economy in dramatic ways. He is also attentive, I imagine, to how AI will affect our sense of reality.
So much of our life is already online and so therefore it will be harder and harder to discern what is real and what is fake, what is true and what is false. So much of our life is virtual, ephemeral, and made up of ever-changing pixels.
But the embodied theology of unity that Augustine provides may very well be a kind of antidote to this crisis. Augustine offers us a true unity rooted in an unchanging God. He offers a Church shaped by mercy which concretely embraces the bodies and souls of the suffering to bring them to Christ. He offers a rich sacramental theology which unites us invisibly, not in the Cloud, but in the Spirit. And, finally, in light of the transhumanist aspirations of the founders of AI who seek to live forever in the machines they have made, Augustine offers us a true vision of transcendence, a true deification, which only the true God can give. And when God is “all in all,” then and only then, will we be truly one in the one.
EDITORIAL NOTE: This essay was originally a talk given for the Lumen Christi Institute’s Ever Ancient, Ever New: The Sources of Pope Leo XIV Lecture Series.
