Introduction – The Didache – Saint Clement of Rome – Saint Ignatius of Antioch – Saint Polycarp of Smyrna – The Epistle of Barnabas – Fragments of Papias – The Shepherd of Hermas – The Epistle to Diognetus – Saint Justin Martyr – Saint Irenaeus of Lyon – Tertullian – Saint Cyprian of Carthage

The Epistle to Diognetus

Virgin with Child, Catacomb of Priscilla (3rd century) — the oldest Marian image

The Epistle to Diognetus is one of the most beautiful and profound documents in all of early Christian literature. Written probably at the end of the second century or the beginning of the third, it is an apologetic letter addressed to a pagan named Diognetus who had asked about the nature of Christianity: in what God do the Christians believe, how do they practice their religion, and why has this new people appeared in the world with such force? The author, whose identity is unknown, responds with an argumentation of extraordinary theological and literary elegance. By reason of its spiritual and thematic closeness to the Apostolic Fathers, tradition has included this letter in the collections of early apostolic writings. Its importance for the debate with Protestantism rests on three points that will be the object of analysis: the doctrine of justification, the nature of the Church, and the apostolic transmission of the Christian mystery.

Justification and the Wondrous Exchange

The debate over justification is, historically, the doctrinal core of the Protestant rupture with the Catholic Church. As we have already noted, the thesis of Luther — that man is justified solely by faith, without works, and that the righteousness of God is imputed to the sinner in a merely forensic manner without transforming him inwardly — clashes with the Catholic teaching that justification is a real transformation of the soul, involving both the grace of God and the free cooperation of man through faith and works. This debate, which might seem to have been introduced only in the sixteenth century, has in reality much older roots; and the Epistle to Diognetus offers on this point a testimony of singular value.

In chapter nine, the author describes in an admirable manner the mystery of redemption and the justification of the sinner. The passage, known in the tradition as the wondrous exchange, is one of the most frequently cited texts in all of patristic literature:

“When our wickedness had reached its height, and it had been clearly shown that its reward, punishment and death, was impending over us; and when the time had come which God had before appointed for manifesting His own kindness and power, how the one love of God, through exceeding regard for men, did not regard us with hatred, nor thrust us away, nor remember our iniquity against us, but showed great long-suffering, and bore with us, He Himself took on Him the burden of our iniquities, He gave His own Son as a ransom for us, the holy One for transgressors, the blameless One for the wicked, the righteous One for the unrighteous, the incorruptible One for the corruptible, the immortal One for those who are mortal. For what other thing was capable of covering our sins than His righteousness? By what other one was it possible that we, the wicked and ungodly, could be justified, than by the only Son of God? O sweet exchange! O unsearchable operation! O benefits surpassing all expectation! That the wickedness of many should be hidden in a single righteous One, and that the righteousness of One should justify many transgressors!” (Epistle to Diognetus 9:2–5)

This extraordinary text has been invoked by both Catholic and Protestant theologians, each interpreting it from their own perspective. Protestants of the Lutheran and Reformed tradition commonly see in it a confirmation of the doctrine of imputed righteousness: Christ takes on our iniquity, and His righteousness is applied to the believer in a forensic manner. However, a careful and contextualized reading of the text does not support that interpretation. First, the central question is not “how is Christ’s righteousness applied to the individual?” but “who can cover our sins?” — and the answer points to the person of the Son of God, not to a juridical mechanism of imputation. Second, the author immediately continues the argument by exhorting the reader to imitate God and to love his neighbor, which presupposes a real transformation of the believer, not a mere juridical declaration of innocence.

Chapter ten, which follows directly upon the cited passage, is luminously clear in this regard. The author exhorts: “If you also desire this faith, you likewise shall receive first of all the knowledge of the Father… Thou shalt both love and admire those that suffer punishment because they will not deny God; you shall condemn the deceit and error of the world when you shall know what it is to live truly in heaven, when you shall despise that which is here esteemed to be death, when you shall fear what is truly death, which is reserved for those who shall be condemned to the eternal fire.” Justification does not culminate in a forensic declaration but in a transformation of life that makes the believer an imitator of God. This is the Catholic doctrine of justifying grace, which not only forgives the sinner but transforms him inwardly through charity.

The Church as the Soul of the World

One of the most celebrated images in all of early Christian literature is the one the author of the Epistle to Diognetus uses to describe the relationship of Christians with the surrounding world: the image of the soul in the body. This comparison, contained in chapters five and six, is not only of great literary beauty, but expresses an ecclesiology — a doctrine of the Church — of enormous relevance for the discussion with Protestantism.

“To sum up all in one word — what the soul is in the body, that are Christians in the world. The soul is dispersed through all the members of the body; and Christians are scattered through all the cities of the world. The soul dwells in the body, yet is not of the body; and Christians dwell in the world, yet are not of the world. The invisible soul is guarded by the visible body; and Christians are known indeed to be in the world, but their godliness remains invisible. The flesh hates the soul, and wars against it, though itself suffering no injury, because it is prevented from enjoying pleasures; the world also hates the Christians, though in nowise injured, because they abjure pleasures. The soul loves the flesh that hates it, and the members; Christians likewise love those that hate them… God has assigned them this illustrious post, which it were unlawful for them to forsake.” (Epistle to Diognetus 6:1–10)

What this text reveals about the vision the first Christians had of the Church is of great importance. The Church is not, for them, an invisible and intangible reality that cannot be identified with any concrete institution — as many Protestant currents tend to hold with the doctrine of the “invisible Church.” On the contrary, Christians are visible in the world, are “scattered through all the cities,” form a recognizable community with a specific worship and a way of life distinct from the world. They are, precisely as an organized and visible community, the soul of the world.

Moreover, the author affirms that “God has assigned them this illustrious post, which it were unlawful for them to forsake.” Belonging to the Christian community, to the visible Church, is neither optional nor secondary for the Christian life: it is a mission entrusted by God that cannot be abandoned. In this sense, the Epistle to Diognetus affirms implicitly what Saint Ignatius of Antioch affirmed explicitly: outside the Church there is no salvation, because it is the Church — and she alone — that is charged with giving spiritual life to the world.

The Mystery Transmitted Through Apostolic Tradition

The last two chapters of the Epistle to Diognetus — chapters eleven and twelve, which many scholars consider a later appendix — address directly the question of the transmission of the Christian mystery. In them, the author presents himself as a disciple of the Apostles and teacher of the nations, and describes the Christian faith as something received from the apostolic tradition and transmitted faithfully from generation to generation. This testimony is of singular importance for the debate over Sola Scriptura.

“The Logos was from the beginning, appeared newly, was found to be old, and is ever born afresh in the hearts of the saints. This is He who is eternal, today counted as a Son, through whom the Church is enriched and grace is unfolded and multiplied among the saints — the grace that gives understanding, that makes mysteries manifest, that announces the appointed times, that rejoices with the faithful.” (Epistle to Diognetus 11:3–5)

The text explicitly associates the transmission of the Christian mystery with the Church: it is she who “is enriched” by the eternal Word, and it is through her that “grace is unfolded and multiplied among the saints.” There is here not the slightest trace of the idea that each believer can access the revealed truth independently and self-sufficiently through the reading of Scripture alone. On the contrary, the revealed truth is a mystery that comes from the Word, is transmitted by the Apostles, and reaches the faithful through the Church that the Word enriches.

The Epistle to Diognetus thus confirms with its testimony what we have found in all the apostolic writings examined throughout this book: the primitive Christian faith is structurally Catholic. Not in the sense that all the dogmas are already explicitly formulated with the technical precision they would attain centuries later — the process of the homogeneous development of doctrine studied in the introduction accounts for that difference — but in the sense that the fundamental structure is the same: a visible, hierarchical, apostolic, sacramental Church, which transmits Revelation through the living tradition, and in which the believer finds the grace that transforms and justifies him. This is the Catholicism that the first Christians lived, and which the great converts of all ages — from Newman to Scott Hahn — have found when studying their writings honestly.