In Illo Uno Unum: Augustine, Pope Leo XIV, and the Longing for Unity

The Longing for Unity: Augustine and the Limits of Philosophy

When Pope Leo XIV chose in illo uno unum (“In the One, we are One”) as his papal motto, he signaled not merely a spiritual aspiration that one might expect from a pope who calls himself a “Son of St. Augustine,” but a profound theological commitment. The phrase comes from St. Augustine’s Exposition on the Psalms, where the bishop of Hippo writes of the Church’s unity in Christ: “Although we Christians are many, in the one Christ we are one” (Exposition on the Book of Psalms,127.2).[1] It is a vision of a unity not accomplished by political harmony, philosophical ascent, or human will, but received as a gift: a unity made possible only in the One through whom all things were made and in whom all things are being restored. Pope Leo’s motto should therefore be seen in light of Augustine’s theology of ecclesial communion, which is always grounded in Christology.[2]

In the classical philosophic tradition, unity is often presented as the fruit of contemplation: the highest good attainable by the few who, through the rigors of philosophy, ascend beyond the world of change and opinion to the unchanging and true. Yet for all its grandeur, this vision leaves most of humanity behind. In his critique of the Neoplatonist Porphyry in City of God X, Augustine brings this problem to the fore. Porphyry, Augustine notes, distinguished two paths of purification: one for the intellectual soul, reserved for philosophers, and another for the lower or “spiritual” soul, accessible through theurgical rites (see City of God X.9–10). This bifurcated way of salvation, Augustine argues, fails in two ways: first because it denies that the whole human person, body and soul together, requires redemption and second because it presumes that the soul can be saved apart from Christ.

For Augustine, this is no minor disagreement but a fundamental difference in anthropology and soteriology. “For what else is the universal way of the soul’s deliverance if not that by which all souls universally are redeemed, and without which, therefore, no soul is redeemed?” Augustine asks (City of God X.32).[3] It seems clear that Pope Leo is thinking along the same lines, since he affirmed in his first homily on May 9th that “Jesus is the Christ, the Son of the living God: the one Savior, who alone reveals the face of the Father.” If salvation is not universal in principle—that is, if it does not address the whole person and extend to all persons—it is not truly salvation. Augustine insists that the Platonists’ reliance on intellect alone results in pride and illusion. The philosophers imagine they ascend by their own power; in truth, they remain bound by sin and error.

This pride, for Augustine, is not merely an intellectual fault. It is a spiritual blindness. Speaking directly of Porphyry, he says: “It was because of his pride that Porphyry did not understand this great mystery: the pride which our true and gracious Redeemer brought low by his own humility when he revealed himself to mortal men clothed in the mortality which he assumed” (City of God X.24).

Augustine’s counterproposal is robust: salvation is not an attainment of those few who are wise enough and talented enough, but a gift received by grace. The Word became flesh to redeem the whole man and, in principle, all men. The theologian’s task is not to sketch a hierarchy of spiritual ways, but to bear witness to the one way open to all: Christ. As he writes, “This is the religion which the universal way of the soul’s deliverance; for no one can be redeemed other than by this way” (City of God X.32).

In this way, Augustine overturns the very premise of Porphyry’s scheme. There is not one path for the many and another for the few. There is one Mediator who took on the whole of human nature so that the whole human race—Jew and Gentile, philosopher and non-philosopher—might be made one. This is no mere metaphor. It is a Christological claim.

The One Mediator and the Totus Christus

For Augustine, the true unity that human beings seek but cannot reach is granted in the mystery of the Incarnation. Human unity cannot be achieved by philosophic exertions or political arrangement; it must be received as a gift. The universal way of salvation is universal not only because it is offered to all, but because it is offered in the One who alone can unite the parts of the human being that have been divided by sin. Our “most true and mighty Purifier and Savior,” Augustine writes, “took upon himself the whole human nature” (City of God X.32).

This “taking of the whole” grounds Augustine’s insistence that Christ alone can save every type of human being—not by addressing each separately, but by uniting all in himself. Porphyry had posited that the intellectual soul might ascend by its own strength, while the lower soul required purification through ritual and contact with lesser spirits. But Augustine sees both paths as futile because they ignore the root problem: not ignorance or pollution from the body, but sin. Christ assumes not simply the highest part of man, nor the lowest, but human nature in its entirety—intellect, soul, and body alike.

Augustine’s own life bears this out. His account in the Confessions describes an ascent beyond the senses, through memory and intellect, until he reaches what he calls id, quod est, that which is. “In the thrust of a trembling glance,” he writes, “[my mind] arrived at that which is” (Confessions VII.17).[4] But he cannot remain there: “I did not stably enjoy my God, but was ravished to you by your beauty, yet soon was torn away from you again by my own weight, and fell again to lower things” (VII.17). The weight is his sinful habit, the “carnal custom” (consuetudo carnalis), that binds him.

What Augustine through philosophy could not achieve on his own, grace later accomplished—but not alone. Augustine’s second ascent is undertaken with his mother Monica, a woman who is devout but unlettered: “And while we were thus talking of his Wisdom and panting for it, with all the effort of our heart we did for one instant attain to touch it” (Confessions IX.10). The sensory metaphor—to touch—is no accident. The ascent culminates not in a flight from the body, but in the presence of One who has assumed it. Moreover, the ascent is not solitary but communal. The fact that Monica, untrained in philosophy, shares in it testifies to the universality of the way opened by Christ.

The unity Augustine here describes is not merely one of affection or mutual intellectual endeavor, let alone a unity forged merely from the custom of false worship. The unity of the Church is the unity of the Body of Christ. As Augustine puts it in his exposition of Psalm 127, “You are therefore many, and you are one; we are many, and we are one. How are we many, and yet one? Because we cling unto him whose members we are.” The Church is totus Christus: Christ the head and we his members. This unity is not of our making. It is the result of our incorporation into Christ, through whom we have access to the Father in one Spirit.

This vision of the totus Christus also addresses the most fundamental social division of the ancient world: that between Jew and Gentile. In City of God, Augustine argues that Christ was sent “to unite in one all peoples,” and that the Church is the “City of God” where this unity is already at work (City of God XV.2, XVII.3). Paul’s claim in Ephesians that Christ “has broken down the dividing wall of hostility” (Eph 2:14) is here taken with full metaphysical seriousness. The reconciliation of Jew and Gentile is not just a historical or moral achievement; it is the outworking of Christ’s assumption of the full human nature and the gathering of all nations into his body.

In this way, in illo uno unum is not a pious abstraction. It is a distillation of Augustine’s deepest convictions about Christology and ecclesiology. True unity is not the work of political consensus (let alone political power), virtue, or philosophical detachment. It is the work of the Incarnate Word, who unites what was divided by sin and calls both philosopher and fisherman to be one in him.

The Church, the City, and Transpolitical Unity

If Christ alone can unite the scattered, then the Church, as his body, is the visible expression of that unity in the world. But for Augustine, the Church’s unity is not political, ethnic, or cultural. It is the sacramental and eschatological unity of the totus Christus, the mystical body of Christ who gathers into himself the many and makes them one.

This unity does not replace the political order, but it does relativize it. Augustine is careful in City of God not to present the Christian emperor as the consummation of political history as some of his contemporaries or near-contemporaries were tempted to. He devotes only brief praise to Constantine and Theodosius, and even then emphasizes their private, not political, virtues. What matters most is not the Christianization of imperial authority but the formation of a people bound together by a common love of God. “Two cities,” he writes, “have been created by two loves: that is, the earthly by love of self extending even to contempt of God, and the heavenly by love of God extending to contempt of self” (City of God XIV.28).

The heavenly city does not abolish the earthly one. Instead, it travels through it as a pilgrim. “For as long as this Heavenly City is a pilgrim on earth,” Augustine says,

She summons citizens of all nations and every tongue, and brings together a society of pilgrims in which no attention is paid to any differences in the customs, laws, and institutions by which earthly peace is achieved or maintained. She does not rescind or destroy these things, however. For whatever differences there are among the various nations, these all tend towards the same end of earthly peace (City of God XIX,17).

This is not a project of assimilation but of transformation through the insertion of believers into the Body of Christ. The Church includes in one body people from every tongue, tribe, and nation—not by removing their differences, but by binding them to one another through their shared, sacramental participation in Christ. Augustine can say that the Church is offered to God as a universal sacrifice, “through the great High Priest who, in his Passion, offered even himself for us in the form of a servant, so that we might be the body of so great a head” (City of God X.6). And, further,

This is the sacrifice of Christians: “We, being many, are one body in Christ.” And this also, as the faithful know, is the sacrifice which the Church continually celebrates in the sacrament of the altar, by which she demonstrates that she herself is offered in the offering that she makes to God (ibid.).

This unity also reveals the limits of politics. Classical political thought had held that law’s authority derived from its divine origin—legitimacy flowed downward from the gods or the sacred founders of a regime. But Christianity, in removing false worship from the center of political life, “secularized” politics in the most profound sense. It made room for a different kind of loyalty, a love that exceeds the city without despising it. As Augustine wrote in a letter,

Therefore, let those who say that the teaching of Christ is opposed to the welfare of the state produce such provincial administrators, such husbands, such wives, such parents, such sons, such masters, such slaves, such kings, such judges, and finally such tax-payers and collectors of public revenue as Christian teaching requires them to be, and then let them dare to say that this teaching is opposed to the welfare of the state, or, rather, let them even hesitate to admit that it is the greatest safety of the state, if it is observed (Letter 138).[5]

Yet the love that binds the Church is not reducible to civil peace. It is more intimate and more enduring. It is a communion in the one Body and one Spirit, sustained by the Eucharist and animated by charity. Augustine says in Tractates on the Gospel of John:

So if you want to understand the body of Christ, listen to the Apostle telling the faithful, “You are the body of Christ and its members” (1 Cor 12:27). So if it’s you that are the body of Christ and its members, it’s the mystery meaning you that has been placed on the Lord’s table; what you receive is the mystery that means you (Sermon 272).[6]

That is why in illo uno unum is not a political slogan or an ecclesiastical platitude. It is a summary of the mystery of salvation. In Christ, the divisions of humanity—Jew and Gentile, elite and ordinary, sinner and saint—are overcome, not by being ignored, but by being healed. The Church becomes one in the One Christ because he alone has taken the many into himself.

In our own time, perhaps Pope Leo might be gearing up to emphasize that it is in the mystical Body of Christ through the grace of incorporation into that body that, in addition to our sins being forgiven, our divisions are overcome. For Augustine and, presumably, Leo, that emphasis would reverberate throughout the entire Christian life: sacramental, liturgical, moral. It might even offer glimpses of the ways in which an Augustinian approach might guide the Church’s synodal way.

Pope Leo’s motto reminds us that the unity of the Church is never our accomplishment. It is Christ’s gift. We do not build the totus Christus; we are incorporated into him. The more we cling to him, the more we find ourselves members of one another. And the more we conform to his humility and charity, the more his glory—that we are made one in the unity of Christ—is made manifest in the world.

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