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

 

The Holy Spirit through the Church is the ladder of ascent to God

But [it has, on the other hand, been shown], that the preaching of the Church is everywhere consistent, and continues in an even course, and receives testimony from the prophets, the apostles, and all the disciples—as I have proved— through [those in] the beginning, the middle, and the end, and through the entire dispensation of God, and that well-grounded system which tends to man’s salvation, namely, our faith; which, having been received from the Church, we do preserve, and which always, by the Spirit of God, renewing its youth, as if it were some precious deposit in an excellent vessel, causes the vessel itself containing it to renew its youth also. For this gift of God has been entrusted to the Church, as breath was to the first created man, for this purpose, that all the members receiving it may be vivified; and the [means of] communion with Christ has been distributed throughout it, that is, the Holy Spirit, the earnest of incorruption, the means of confirming our faith, and the ladder of ascent to God.

…For where the Church is, there is the Spirit of God; and where the Spirit of God is, there is the Church, and every kind of grace; but the Spirit is truth.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XXIV

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 only true Church of the Apostles

Call to mind then, the things which I have stated in the two preceding books, and, taking these in connection with them, thou shalt have from me a very copious refutation of all the heretics; and faithfully and strenuously shalt thou resist them in defense of the only true and life-giving faith, which the Church has received from the apostles and imparted to her sons. For the Lord of all gave to His apostles the power of the Gospel, through whom also we have known the truth, that is, the doctrine of the Son of God; to whom also did the Lord declare: “He that hears you, hears Me; and he that despises you, despises Me, and Him that sent Me.”

Irenaeus of Lyons
Against Heresies, Book III
Chapter I

Faith is received through the Church

But such spirits as are commanded by these men, and speak when they desire it, are earthly and weak, audacious and impudent, sent forth by Satan for the seduction and perdition of those who do not hold fast that well-compacted faith which they received at first through the Church.

Irenaeus of Lyons
Against Heresies Book I
Chapter XIII

No one adds or removes from the tradition of the Church

Nor will any one of the rulers in the Churches, however highly gifted he may be in point of eloquence, teach doctrines different from these (for no one is greater than the Master); nor, on the other hand, will he who is deficient in power of expression inflict injury on the tradition. For the faith being ever one and the same, neither does one who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little diminish it.

Irenaeus of Lyons
Against Heresies Book I
Chapter X