Chapter 3.—In What Way the Church, Through Possessing the Tradition of the Apostles, and the Preaching of the Truth, Has Been Preserved from These Heresies. Proof of This in the Church of Rome.

1. It is within the power of all, therefore, in every Church, who may wish to see the truth, to contemplate clearly the tradition of the apostles manifested throughout the whole world; and we are in a position to reckon up in detail those who were by the apostles instituted bishops in the Churches, and [to demonstrate] the uninterrupted succession of these men to our own times: those who neither taught nor knew of anything like what these [heretics] rave about. For if the apostles had known hidden mysteries, which they were in the habit of imparting to the perfect apart and privily from the rest, they would have delivered them especially to those to whom they were also committing the Churches themselves. For these men they desired to be very perfect and blameless in all things, whom also they were leaving behind as their successors, delivering up their own place of government to these men; which men, if they discharged their functions honestly, would be a great boon [to the Church], but if they fell away, would prove the direst calamity.

2. Since, however, it would be very tedious, in such a volume as this, to reckon up the successions of all the Churches, we do put to confusion all those who, in whatever manner, whether by an evil self-pleasing, by vainglory, or by blindness and perverse opinion, assemble in unauthorized meetings; [we do this, I say,] by indicating that tradition derived from the apostles, of the very great, the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul; as also [by pointing out] the faith preached to men, which comes down to our time by means of the successions of the bishops. For it is a matter of necessity that every Church should agree with this Church, on account of its pre-eminent authority, that is, the faithful everywhere, inasmuch as the apostolic tradition has been preserved continuously by those [faithful men] who exist everywhere.

3. The blessed apostles, then, having founded and built up the Church, committed into the hands of Linus the office of the episcopate. Of this Linus, Paul makes mention in the Epistles to Timothy. To him succeeded Anacletus; and after him, in the third place from the apostles, Clement was allotted the episcopate. 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. Nor was he alone [in this]; for there were many still remaining who had received instruction from the apostles. In the time of this Clement, no small dissension having occurred among the brethren at Corinth, the Church in Rome dispatched a most powerful letter to the Corinthians, exhorting them to peace and renewing their faith. The Ephesians, indeed, did address him, the father of the said letter, from the fact that Paul had founded that Church. In this Epistle, Clement plainly indicates that he held the tradition of the apostles to be the means by which the Church could be kept from falling into error. For he says: “You therefore, who laid the foundation of this sedition, be subject to the presbyters, and receive correction so as to repent, bending your knees of your hearts.” For Clement thus wrote to the Corinthians, after the apostles (Peter and Paul) had departed this life. To this Clement succeeded Evaristus. Alexander followed Evaristus; then, sixth from the apostles, Sixtus was appointed; after him, Telephorus, who was gloriously martyred; then from him, 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. And this is most abundant proof that there is one and the same vivifying faith, which has been preserved in the Church from the apostles until now, and handed down in truth.

Irenaeus, Bishop of Lyons (c. 130-c. 200). The great theologian of the second century, and the first to formulate a comprehensive system of Christian doctrine, was born in Asia Minor, probably at Smyrna. He relates that in his youth he listened to the preaching of St Polycarp, who was himself a disciple of St John the Evangelist; this direct link with the Apostolic age was a source of great pride and authority for Irenaeus. It is not known when he left Smyrna, but by c. 177 he was a priest in the Church of Lyons. In that year he was sent to Rome as a legate from the local confessors to Pope Eleutherius, bearing a letter concerning the Montanist controversy. While he was away, the persecution of Marcus Aurelius broke out in Lyons, and the aged Bishop Pothinus was martyred. On his return, Irenaeus succeeded Pothinus as Bishop.

His episcopate saw a period of intense theological controversy, particularly the rise of Gnosticism, which threatened to undermine the very foundations of Christian belief. Irenaeus devoted his life to combating these heresies. His principal work, 'Adversus Haereses' (Against Heresies), written c. 180-185, is a monumental refutation of Gnostic doctrines. In this five-book treatise, he systematically exposed the absurdities and contradictions of various Gnostic systems, particularly those of Valentinus and Marcion. He argued that Gnosticism, with its dualistic view of God (a perfect, unknowable God and a lesser, evil demiurge responsible for creation), its denial of the Incarnation, and its claim to secret knowledge (gnosis), was fundamentally incompatible with the apostolic tradition.

Irenaeus’s method of refutation was twofold: first, he demonstrated the novelty and internal inconsistencies of Gnostic teachings; second, and more importantly, he presented a coherent exposition of orthodox Christian doctrine, grounded in the undisputed apostolic tradition. He emphasized the unity of God as Creator and Redeemer, the reality of Christ’s Incarnation as truly God and truly man, and the bodily resurrection. He insisted that the true faith was publicly accessible, handed down through the succession of bishops in the Churches founded by the apostles, especially Rome. This appeal to apostolic succession and tradition became a foundational principle for subsequent Christian theology.

Central to Irenaeus’s theology is his concept of 'recapitulation' (anakephalaiosis). He taught that Christ, as the new Adam, 'recapitulated' or summed up all of human history and creation in Himself, reversing the disobedience of the first Adam. Through His Incarnation, life, death, and resurrection, Christ restored humanity to communion with God and brought about the renewal of creation. This concept highlights the cosmic scope of salvation and the unity of God’s plan throughout history.

Irenaeus also played a crucial role in mediating the Quartodeciman controversy, a dispute over the date of Easter. While Pope Victor I of Rome was prepared to excommunicate the Asian churches for their adherence to an older tradition of celebrating Easter on the 14th of Nisan, Irenaeus, although himself following the Roman practice, wrote to Victor urging patience and understanding. He reminded the Pope that diversity in custom should not break the unity of faith, effectively averting a schism. This intervention exemplifies his deep commitment to the unity of the Church.

Though ancient martyrologies record his martyrdom, the details are uncertain, and there is no contemporary evidence to confirm it. He is believed to have died around 200 AD. His feast day is celebrated on June 28. In recognition of his profound theological contributions and his tireless efforts for Christian unity, Pope Francis declared him a Doctor of the Church on January 21, 2022, bestowing upon him the title 'Doctor of Unity.' Irenaeus remains a towering figure in early Christian thought, whose clear articulation of orthodox doctrine and steadfast defense of apostolic tradition laid crucial groundwork for the development of Christian theology.