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

Saint Irenaeus of Lyon

Saint Irenaeus of Lyon, bishop and martyr — traditional icon

Saint Irenaeus was born around the year 130, probably in Asia Minor. In his youth he was a disciple of Saint Polycarp of Smyrna, who had himself been a disciple of the Apostle John. He became bishop of Lyon around 178 and died a martyr around 202. His principal work, Adversus Haereses (Against Heresies), in five books, is the first great system of Catholic theology and the most comprehensive refutation of early Gnosticism. In it, Irenaeus systematically employs the arguments of apostolic succession, the tradition of the Church, and the primacy of Rome to demonstrate the authenticity of the Catholic faith against the innovations of heretics. His testimony is of exceptional value because, through Polycarp, he represents a direct chain of transmission reaching back to the Apostle John.

Apostolic Succession and the Primacy of Rome

The central argument of Irenaeus’s Adversus Haereses against the Gnostics is the argument from apostolic succession: the true faith can be identified because it has been transmitted in an unbroken manner from the Apostles, through a chain of bishops whose list can be publicly and verifiably recited. The Gnostics, by contrast, claimed to possess secret revelations with no verifiable connection to the Apostles. This same argument applies directly to Protestantism: Protestant denominations cannot trace any unbroken line of apostolic succession back to the Apostles. Irenaeus, for his part, provides the list of the bishops of Rome up to his own day as proof of authenticity:

“The blessed apostles, then, having founded and built up the Church, committed into the hands of Linus the office of the episcopate… To him succeeded Anacletus; and after him, in the third place from the apostles, Clement was allotted the bishopric. This man, as he had seen the blessed apostles and had been conversant with them, might be said to have the preaching of the apostles still echoing in his ears and their traditions before his eyes… To this Clement there succeeded Evaristo. Alexander followed Evaristo; then, sixth from the apostles, Sixtus was appointed; after him, Telephorus, who was gloriously martyred; then Hyginus; after him, Pius; then after him, Anicetus. Soter having succeeded Anicetus, Eleutherius does now, in the twelfth place from the apostles, hold the inheritance of the episcopate. In this order, and by this succession, the ecclesiastical tradition from the apostles, and the preaching of the truth, have come down to us.” (Irenaeus of Lyon, Adversus Haereses 3:3:3)

This list of the bishops of Rome, from Peter and Paul to his own day, is the historical argument par excellence against any claim to renew Christianity from outside apostolic succession. The true faith is identified by its verifiable continuity with the Apostles. Irenaeus further explains why it is precisely the Church of Rome that serves as the primary reference of apostolic authenticity:

“For it is a matter of necessity that every church should agree with this Church, on account of its preeminent authority — that is, the faithful everywhere — in which the apostolic tradition has always been preserved by those who exist everywhere.” (Irenaeus of Lyon, Adversus Haereses 3:3:2)

The Church of Rome, founded by Peter and Paul, possesses a special authority (potiorem principalitatem) to which the whole Church must have recourse as a criterion of apostolic authenticity. This primacy is not a medieval invention: Irenaeus affirms it in the second century, as something known and recognized by all. This is the doctrine that Protestantism would reject fifteen centuries later.

Tradition versus Sola Scriptura

Irenaeus also provides one of the strongest arguments against the Protestant doctrine of Sola Scriptura. In book three of the Adversus Haereses, before proceeding to demonstrate the apostolic faith from the Scriptures, Irenaeus points out that the oral tradition of the Church would by itself be sufficient to establish the true faith, even if the Scriptures did not exist:

“For how should it be if the apostles themselves had not left us writings? Would it not be necessary, in that case, to follow the course of the tradition which they handed down to those to whom they did commit the Churches? To which course many nations of those barbarians who believe in Christ do assent, having salvation written in their hearts by the Spirit, without paper and ink, and, carefully preserving the ancient tradition.” (Irenaeus of Lyon, Adversus Haereses 3:4:1)

The argument of Irenaeus completely inverts the logic of Sola Scriptura. It is not Scripture that authorizes the Church, but the Church that possesses the apostolic tradition of which Scripture is a part. Entire peoples have received the faith without any access to a written text, and yet believe correctly because they have received the oral tradition of the Apostles. This same logic is what the Catholic Church has always defended: Scripture and Tradition are two channels of the single apostolic Revelation, and neither of them can be correctly understood without the other.

But the most devastating argument against Sola Scriptura is not the one Irenaeus formulates explicitly, but the one that flows from the very logic of his position: if Scripture is the sole rule of faith, how do we know which books belong to Scripture? The New Testament canon was not obvious from the beginning. During the first centuries Christians debated whether books such as Revelation, the Letter to the Hebrews, or Second Peter were inspired, and whether texts like the Gospel of Peter or the Didache should be included. It was the Church, guided by her living apostolic Tradition, that discerned and defined which books were inspired. Without that ecclesial Tradition, there is no Bible — or, more precisely, there is no way of knowing which books make up the Bible. The Protestant who rejects the Church’s Tradition as a source of revelation while accepting the Bible as the sole rule of faith does so paradoxically by virtue of that very Tradition he rejects.

But the problem of the canon is not limited to the New Testament. Protestantism also rejects seven books of the Old Testament that the Catholic Church has always included in Sacred Scripture: Tobit, Judith, 1 and 2 Maccabees, Wisdom, Sirach, and Baruch — called “deuterocanonical” by Catholics and “apocryphal” by Protestants. Luther excluded them on the basis of the rabbinic Hebrew canon consolidated after the destruction of the Temple in the first century; but the early Fathers of the Church used the Septuagint — the Greek translation of the Old Testament that was the Bible of the Apostles and of the entire primitive Church — and treated those books as part of Sacred Scripture. Saint Clement of Rome, in chapter 55 of his Letter to the Corinthians, cites the book of Judith as an example of heroic virtue with no indication that he considers it a text of lesser authority: “The blessed Judith, when her city was besieged, asked the elders that they would permit her to go out into the camp of the strangers…” (Clement of Rome, Letter to the Corinthians 55:4–5). This citation treated as Scripture by the very successor of Peter confirms that the removal of seven books of the Old Testament carried out by the Reformation was a novelty without precedent in the history of Christianity — not the recovery of an original canon. Here too the Protestant position clashes head-on with the testimony of the most ancient Church.

Irenaeus, for his part, not only defends Apostolic Tradition as a source of revelation: he also articulates what he calls the “rule of truth” (kanon tès alètheias) or “rule of faith” (regula fidei) — a summary of the content of the apostolic faith transmitted from generation to generation from the time of the Apostles. This rule is not something added to Scripture from the outside: it is the hermeneutical key without which Scripture cannot be correctly understood. Irenaeus observes that the Gnostics use the same biblical texts as the Catholics but arrive at completely different conclusions precisely because they interpret them outside the rule of faith: “Collecting a set of expressions and names scattered here and there in Scripture, they twist them, as we have already said, from a natural to a non-natural sense.” (Irenaeus of Lyon, Adversus Haereses 1:8:1). Scripture without Tradition is a text that every interpreter can mold as he pleases: the history of Protestantism, with its thousands of denominations all arising from the same biblical text and arriving at mutually incompatible conclusions, is the most eloquent empirical proof that Irenaeus was right.

The Eucharist as Sacrifice

The eucharistic doctrine of Irenaeus complements the testimony of Justin and deepens it in a dimension that Protestantism explicitly rejects: the sacrificial character of the Eucharist. For the reformers, the Mass as sacrifice is a blasphemy against the unique and unrepeatable sacrifice of Christ on the Cross. For the Catholic Church, the Mass is the sacramental re-presentation of that same unique sacrifice, not a repetition. Irenaeus, in the second century, already knows this doctrine:

“We offer to Him His own, announcing consistently the fellowship and union of the flesh and Spirit. For as the bread, which is produced from the earth, when it receives the invocation of God, is no longer common bread, but the Eucharist, consisting of two realities, earthly and heavenly; so also our bodies, when they receive the Eucharist, are no longer corruptible, having the hope of the resurrection to eternity.” (Irenaeus of Lyon, Adversus Haereses 4:18:5)

The Eucharist, for Irenaeus, is an offering to God: the bread and wine, upon receiving the invocation (the epiclesis, the consecratory prayer), cease to be common bread and wine and become something composed of two realities — one earthly and one heavenly. This is the doctrine of transubstantiation expressed in the language of the second century: what was previously bread has been converted in its substance into the Body of Christ — the external accidents of bread, its appearances, remain, but the profound reality underlying them has been transformed into food of immortality.

Baptism and the Salvation of Children

One of the most heated debates between Catholicism and broad sectors of evangelical and Baptist Protestantism is that of infant baptism. For these traditions, baptism is an act of personal confession of faith that can only be performed when the person has reached the age of reason; baptizing an infant would therefore be an act devoid of theological meaning. This position, however, lacks any support in the writings of the primitive Church and clashes head-on with the testimony of Saint Irenaeus of Lyon, formed in the apostolic tradition of Asia Minor.

In book two of the Adversus Haereses, Irenaeus develops a christological reflection of notable depth: Christ, he argues, lived and sanctified every stage of human life, from infancy to old age, in order to redeem the whole man. It is in this context that the most important passage in all of primitive patristic literature on infant baptism appears:

“He came to save all through means of Himself — all, I say, who through Him are born again to God — infants, and children, and boys, and youths, and old men.” (Irenaeus of Lyon, Adversus Haereses 2:22:4)

The decisive phrase is “those who through Him are born again to God.” The verb “to be born again” (renasci in Irenaeus’s Latin) is the New Testament technical term for baptism, directly rooted in the words of Christ to Nicodemus: “unless one is born of water and the Spirit, he cannot enter the kingdom of God” (John 3:5). Irenaeus does not say that adults are reborn but infants are not: he affirms that Christ came to save all who are reborn, and enumerates without exception every stage of human life, explicitly including infants. This presupposes that infants were baptized in the Church of the second century as an ordinary practice, and that through that baptism they were born again to God. The chain of tradition of Irenaeus reaches directly back to the Apostles through Saint Polycarp and Saint John: it is hard to suppose this was a late innovation.

The Protestant exegete Joachim Jeremias, in his rigorous study on infant baptism in the early centuries, concluded that there exists no primitive patristic testimony that condemns or questions the practice of baptizing children, and that the evidence points to its apostolic origin. The silence of the Fathers in the face of any possible Baptist objection is itself an eloquent argument: no Christian writer of the first and second centuries considers it necessary to defend the baptism of children because no one in the Church questioned it. When the debate arose centuries later, it was always as a novelty, never as the recovery of something primitive.