Differences in fasting customs between East and West are accepted

Thus, in fact, the difference [in observing] the fast establishes the harmony of [our common] faith. For neither could Anicetus persuade Polycarp to forego the observance [in his own way], inasmuch as these things had been always [so] observed by John the disciple of our Lord, and by other apostles with whom he had been conversant; nor, on the other hand, could Polycarp succeed in persuading Anicetus to keep [the observance in his way], for he maintained that he was bound to adhere to the usage of the presbyters who preceded him.

…so that they parted in peace one from the other, maintaining peace with the whole Church, both those who did observe [this custom] and those who did not.

Irenaeus of Lyons
Lost Writings
Fragment III

Irenaeus learned from Polycarp as a boy

For, while I was yet a boy, I saw thee in Lower Asia with Polycarp, distinguishing thyself in the royal court, and endeavouring to gain his approbation. For I have a more vivid recollection of what occurred at that time than of recent events (inasmuch as the experiences of childhood, keeping pace with the growth of the soul, become incorporated with it); so that I can even describe the place where the blessed Polycarp used to sit and discourse— his going out, too, and his coming in—his general mode of life and personal appearance, together with the discourses which he delivered to the people; also how he would speak of his familiar intercourse with John, and with the rest of those who had seen the Lord; and how he would call their words to remembrance. Whatsoever things he had heard from them respecting the Lord, both with regard to His miracles and His teaching, Polycarp having thus received [information] from the eye-witnesses of the Word of life, would recount them all in harmony with the Scriptures. These things, through, God’s mercy which was upon me, I then listened to attentively, and treasured them up not on paper, but in my heart; and I am continually, by God’s grace, revolving these things accurately in my mind.

Irenaeus of Lyons
Lost Writings
Fragment II

 

The catholicity of the Church

But the path of those belonging to the Church circumscribes the whole world, as possessing the sure tradition from the apostles, and gives unto us to see that the faith of all is one and the same, since all receive one and the same God the Father, and believe in the same dispensation regarding the incarnation of the Son of God, and are cognizant of the same gift of the Spirit, and are conversant with the same commandments, and preserve the same form of ecclesiastical constitution, and expect the same advent of the Lord, and await the same salvation of the complete man, that is, of the soul and body.

…For the Church preaches the truth everywhere, and she is the seven-branched candlestick which bears the light of Christ.

Irenaeus of Lyons
Against Heresies, Book V
Chapter XX

Biblical interpretation must be done in light of catholic tradition

But here some one perhaps will ask, Since the canon of Scripture is complete, and sufficient of itself for everything, and more than sufficient, what need is there to join with it the authority of the Church’s interpretation? For this reason—because, owing to the depth of Holy Scripture, all do not accept it in one and the same sense, but one understands its words in one way, another in another; so that it seems to be capable of as many interpretations as there are interpreters. For Novatian expounds it one way, Sabellius another, Donatus another, Arius, Eunomius, Macedonius, another, Photinus, Apollinaris, Priscillian, another,Iovinian, Pelagius, Celestius, another, lastly, Nestorius another. Therefore, it is very necessary, on account of so great intricacies of such various error, that the rule for the right understanding of the prophets and apostles should be framed in accordance with the standard of Ecclesiastical and Catholic interpretation.

Moreover, in the Catholic Church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense Catholic, which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient definitions and determinations of all, or at the least of almost all priests and doctors.

St. Vincent of Lerins
Commonitory
Chapter 2:5-6

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

 

Christ’s descent to the dead

It was for this reason, too, that the Lord descended into the regions beneath the earth, preaching His advent there also, and [declaring] the remission of sins received by those who believe in Him. Now all those believed in Him who had hope towards Him, that is, those who proclaimed His advent, and submitted to His dispensations, the righteous men, the prophets, and the patriarchs, to whom He remitted sins in the same way as He did to us, which sins we should not lay to their charge, if we would not despise the grace of God. For as these men did not impute unto us (the Gentiles) our transgressions, which we wrought before Christ was manifested among us, so also it is not right that we should lay blame upon those who sinned before Christ’s coming.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVII

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

 

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

Barbarians believe in Christ in the absence of scripture

…many nations of those barbarians who believe in Christ do assent, having salvation written in their hearts by the Spirit, without paper or ink, and, carefully preserving the ancient tradition

…Those who, in the absence of written documents, have believed this faith, are barbarians,

…If any one were to preach to these men the inventions of the heretics, speaking to them in their own language, they would at once stop their ears, and flee as far off as possible, not enduring even to listen to the blasphemous address. Thus, by means of that ancient tradition of the apostles, they do not suffer their mind to conceive anything of the [doctrines suggested by the] portentous language of these teachers, among whom neither Church nor doctrine has ever been established.

Irenaeus of Lyons
Against Heresies, Book III
Chapter IV