True doctrine is found through the succession of the ancient Church

True knowledge is [that which consists in] the doctrine of the apostles, and the ancient constitution of the Church throughout all the world, and the distinctive manifestation of the body of Christ according to the successions of the bishops, by which they have handed down that Church which exists in every place, and has come even unto us, being guarded and preserved without any forging of Scriptures, by a very complete system of doctrine, and neither receiving addition nor [suffering] curtailment [in the truths which she believes]; and [it consists in] reading [the word of God] without falsification, and a lawful and diligent exposition in harmony with the Scriptures, both without danger and without blasphemy; and [above all, it consists in] the pre-eminent gift of love, which is more precious than knowledge, more glorious than prophecy, and which excels all the other gifts [of God].

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXXIII

 

The Apostolic faith is through succession and is the key to the Scriptures

Where, therefore, the gifts of the Lord have been placed, there it behoves us to learn the truth, [namely,] from those who possess that succession of the Church which is from the apostles, and among whom exists that which is sound and blameless in conduct, as well as that which is unadulterated and incorrupt in speech. For these also preserve this faith of ours in one God who created all things; and they increase that love [which we have] for the Son of God, who accomplished such marvelous dispensations for our sake: and they expound the Scriptures to us without danger, neither blaspheming God, nor dishonoring the patriarchs, nor despising the prophets.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVII

Gatherings outside of Apostolic succession should be viewed with suspicion

Wherefore it is incumbent to obey the presbyters who are in the Church,—those who, as I have shown, possess the succession from the apostles; those who, together with the succession of the episcopate, have received the certain gift of truth, according to the good pleasure of the Father. But [it is also incumbent] to hold in suspicion others who depart from the primitive succession, and assemble themselves together in any place whatsoever, [looking upon them] either as heretics of perverse minds, or as schismatics puffed up and self-pleasing, or again as hypocrites, acting thus for the sake of lucre and vainglory.

…But those who cleave asunder, and separate the unity of the Church, [shall] receive from God the same punishment as Jeroboam did.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVI

 

Doctrinal disputes are resolved by the ancient Churches founded by the Apostles

Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient Churches with which the apostles held constant intercourse, and learn from them what is certain and clear in regard to the present question? 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?

Irenaeus of Lyons
Against Heresies, Book III
Chapter IV

 

The ecclesiastical tradition of the apostles is handed down

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 of Lyons
Against Heresies, Book III
Chapter III

The Roman tradition was established by Peter and Paul and succeeded by bishops

…[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 apostolical tradition has been preserved continuously by those [faithful men] who exist everywhere.

Irenaeus of Lyons
Against Heresies, Book III
Chapter III

The truth is found in the tradition of the Apostles instituted in their Churches

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 those who were by the apostles instituted bishops in the Churches, and [to demonstrate] the succession of these men to our own times; those who neither taught nor knew of anything like what these [heretics] rave about.

…For they were desirous that these men should 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 should fall away, the direst calamity.

Irenaeus of Lyons
Against Heresies, Book III
Chapter III

The tradition of the Apostles is preserved by Apostolic Succession

But, again, when we refer them to that tradition which originates from the apostles, [and] which is preserved by means of the succession of presbyters in the Churches, they object to tradition, saying that they themselves are wiser not merely than the presbyters, but even than the apostles, because they have discovered the unadulterated truth.

…It comes to this, therefore, that these men do now consent neither to Scripture nor to tradition.

Irenaeus of Lyons
Against Heresies, Book III
Chapter II